Make the missing symbols script more useful.

If something's in POSIX, we don't need to show it in the glibc list.

If something's not implemented by glibc, we don't need to show it in the
POSIX list (because it's probably either a macro or obsolete).

Change-Id: Ied0f8d97d3fffb280c22e9cdf6782430d776c02f