Further work on animated GIF export: XDG Pictures

Using XDG's user dir settings to determine where pictures are
stored for a user (e.g., "~/Pictures" -- used as a fallback).
May be overridden using "--exportdir".

Also, while I was updating some docs, replace references to
"Mac OS X" with "macOS", the new name of that OS these days.

src/parse.h

@@ -22,6 +22,7 @@ struct cfginfo
   const char *disable_stamp_controls;
   const char *dont_do_xor;
   const char *dont_load_stamps;
+  const char *exportdir;
   const char *fullscreen;
   const char *grab_input;
   const char *hide_cursor;