Add more const

While I'm not very fond of "const", it's important for declarations
(it decides whether a symbol is emitted in a read-only or read/write
section). Fix all these cases, so we have writeable global data only
when we really need.

player/configfiles.c

@@ -207,7 +207,7 @@ exit:
     return res;
 }
 
-static const char *backup_properties[] = {
+static const char *const backup_properties[] = {
     "options/osd-level",
     //"loop",
     "options/speed",