scripting: add a way to run sub processes as "scripts"

This is just a more convenient way to start IPC client scripts per mpv
instance.

Does not work on Windows, although it could if the subprocess and IPC
parts are implemented (and I guess .exe/.bat suffixes are required).
Also untested whether it builds on Windows. A lot of other things are
untested too, so don't complain.

input/input.h

@@ -213,10 +213,20 @@ void mp_input_sdl_gamepad_add(struct input_ctx *ictx);
 
 struct mp_ipc_ctx;
 struct mp_client_api;
+struct mpv_handle;
 
 // Platform specific implementation, provided by ipc-*.c.
 struct mp_ipc_ctx *mp_init_ipc(struct mp_client_api *client_api,
                                struct mpv_global *global);
+// Start a thread for the given handle and return a socket in out_fd[0] that
+// is served by this thread. If the FD is not full-duplex, then out_fd[0] is
+// the user's read-end, and out_fd[1] the write-end, otherwise out_fd[1] is set
+// to -1.
+//  returns:
+//      true: out_fd[0] and out_fd[1] are set, ownership of h is transferred
+//      false: out_fd are not touched, caller retains ownership of h
+bool mp_ipc_start_anon_client(struct mp_ipc_ctx *ctx, struct mpv_handle *h,
+                              int out_fd[2]);
 void mp_uninit_ipc(struct mp_ipc_ctx *ctx);
 
 // Serialize the given mpv_event structure to JSON. Returns an allocated string.

input/ipc-dummy.c

@@ -8,6 +8,12 @@ struct mp_ipc_ctx *mp_init_ipc(struct mp_client_api *client_api,
     return NULL;
 }
 
+bool mp_ipc_start_anon_client(struct mp_ipc_ctx *ctx, struct mpv_handle *h,
+                              int out_fd[2])
+{
+    return false;
+}
+
 void mp_uninit_ipc(struct mp_ipc_ctx *ctx)
 {
 }

input/ipc-unix.c

@@ -58,7 +58,7 @@ struct client_arg {
     struct mp_log *log;
     struct mpv_handle *client;
 
-    char *client_name;
+    const char *client_name;
     int client_fd;
     bool close_client_fd;
 
@@ -215,9 +215,11 @@ done:
     return NULL;
 }
 
-static void ipc_start_client(struct mp_ipc_ctx *ctx, struct client_arg *client)
+static bool ipc_start_client(struct mp_ipc_ctx *ctx, struct client_arg *client,
+                             bool free_on_init_fail)
 {
-    client->client = mp_new_client(ctx->client_api, client->client_name);
+    if (!client->client)
+        client->client = mp_new_client(ctx->client_api, client->client_name);
     if (!client->client)
         goto err;
 
@@ -227,16 +229,19 @@ static void ipc_start_client(struct mp_ipc_ctx *ctx, struct client_arg *client)
     if (pthread_create(&client_thr, NULL, client_thread, client))
         goto err;
 
-    return;
+    return true;
 
 err:
-    if (client->client)
-        mpv_destroy(client->client);
+    if (free_on_init_fail) {
+        if (client->client)
+            mpv_destroy(client->client);
 
-    if (client->close_client_fd)
-        close(client->client_fd);
+        if (client->close_client_fd)
+            close(client->client_fd);
+    }
 
     talloc_free(client);
+    return false;
 }
 
 static void ipc_start_client_json(struct mp_ipc_ctx *ctx, int id, int fd)
@@ -246,11 +251,37 @@ static void ipc_start_client_json(struct mp_ipc_ctx *ctx, int id, int fd)
         .client_name = talloc_asprintf(client, "ipc-%d", id),
         .client_fd   = fd,
         .close_client_fd = true,
-
         .writable = true,
     };
 
-    ipc_start_client(ctx, client);
+    ipc_start_client(ctx, client, true);
+}
+
+bool mp_ipc_start_anon_client(struct mp_ipc_ctx *ctx, struct mpv_handle *h,
+                              int out_fd[2])
+{
+    int pair[2];
+    if (socketpair(AF_UNIX, SOCK_STREAM, 0, pair))
+        return false;
+
+    struct client_arg *client = talloc_ptrtype(NULL, client);
+    *client = (struct client_arg){
+        .client = h,
+        .client_name = mpv_client_name(h),
+        .client_fd   = pair[1],
+        .close_client_fd = true,
+        .writable = true,
+    };
+
+    if (!ipc_start_client(ctx, client, false)) {
+        close(pair[0]);
+        close(pair[1]);
+        return false;
+    }
+
+    out_fd[0] = pair[0];
+    out_fd[1] = -1;
+    return true;
 }
 
 static void ipc_start_client_text(struct mp_ipc_ctx *ctx, const char *path)
@@ -292,7 +323,7 @@ static void ipc_start_client_text(struct mp_ipc_ctx *ctx, const char *path)
         .writable = writable,
     };
 
-    ipc_start_client(ctx, client);
+    ipc_start_client(ctx, client, true);
 }
 
 static void *ipc_thread(void *p)

input/ipc-win.c

@@ -335,6 +335,12 @@ static void ipc_start_client_json(struct mp_ipc_ctx *ctx, int id, HANDLE h)
     ipc_start_client(ctx, client);
 }
 
+bool mp_ipc_start_anon_client(struct mp_ipc_ctx *ctx, struct mpv_handle *h,
+                              int out_fd[2])
+{
+    return false;
+}
+
 static void *ipc_thread(void *p)
 {
     // Use PIPE_TYPE_MESSAGE | PIPE_READMODE_BYTE so message framing is