Read GSS configuration files with mtime 0

Read GSS configuration files with mtime 0

There is at least one case (with flatpaks) where configuration files
in the special read-only /etc all have an mtime of 0.  Using an
initial last modified time of 0 in g_initialize.c causes these files
to never be read.

Change the initial high value to the be the "invalid" value
(time_t)-1.  Since the C and POSIX standards do not require time_t to
be signed, special-case the checks in load_if_changed() and
updateMechList() to treat all mod times as newer than -1.

[ghudson@mit.edu: edited commit message; slightly modified approach]

ticket: 9060 (new)
target_version: 1.20
tags: pullup