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.gperf

@@ -112,6 +112,7 @@ datadir,             MULTI(datadir)
 disablescreensaver,  POSBOOL(disable_screensaver)
 dontgrab,            NEGBOOL(grab_input)
 dontmirrorstamps,    NEGBOOL(mirrorstamps)
+exportdir,           MULTI(exportdir)
 fancycursors,        NEGBOOL(no_fancy_cursors)
 fullscreen,          MULTI(parsertmp_fullscreen_native)
 grab,                POSBOOL(grab_input)