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.
    
    Signed-off-by: Julien Rische <jrische@redhat.com>