Fix prompt under Windows.

The fix is obviously a hack caused by that NOT_A_WCHAR
doesn't fit in wchar_t. Better fix would be nice.
avoid symlink attacks in __fish_print_packages and spawning fishd

 * use $XDG_CACHE_HOME for __fish_print_packages completion caches
 * when starting fishd, redirect fishd output to /dev/null, not a
   predictable path

Fix for CVE-2014-3219.

Closes #1440.
Hopefully better fix for prompt under Windows. Things that may store
NOT_A_WCHAR must be wint_t, not wchar_t
Merge branch 'master' into death_of_fishd
Correct a comment and clean up code that listens for universal notifier
changes