lib: canonicalize pull_dos_date3()

lib: canonicalize pull_dos_date3()

Returns 0xFFFFFFFF as (time_t)-1. This avoids misenterpreting 0xFFFFFFFF as a
valid time_t value (0xFFFFFFFF = Sun 07 Feb 2106 06:28:15 AM GMT) on 64-bit
platforms where time_t is 64-bit.

Currently direct and indirect callers of pull_dos_date3() rely on the fact that
the resulting time_t is checked with null_time() which also checks for
0xFFFFFFFF as sentinel value amongst 0 and -1:

        return t == 0 ||
                t == (time_t)0xFFFFFFFF ||
                t == (time_t)-1;

By returning -1 instead of 0xFFFFFFFF, callers can safely pass the result to
unix_to_nt_time() which *doesn't* check for 0xFFFFFFFF, only -1.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=7771

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>