lua: subprocess: move to osdep/subprocess-{win,posix}.c

The subprocess code was already split into fairly general functions,
separate from the Lua code. It's getting pretty big though, especially
the Windows-specific parts, so move it into its own files.

osdep/subprocess.h

@@ -0,0 +1,30 @@
+/*
+ * This file is part of mpv.
+ *
+ * mpv is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * mpv is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with mpv.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef MP_SUBPROCESS_H_
+#define MP_SUBPROCESS_H_
+
+#include "stream/stream.h"
+
+typedef void (*subprocess_read_cb)(void *ctx, char *data, size_t size);
+
+// Start a subprocess. Uses callbacks to read from stdout and stderr.
+int mp_subprocess(char **args, struct mp_cancel *cancel, void *ctx,
+                  subprocess_read_cb on_stdout, subprocess_read_cb on_stderr,
+                  char **error);
+
+#endif