smbd: fix handling of sentinel timestamp values

smbd: fix handling of sentinel timestamp values

This implements two core changes:

* use NTTIME instead of struct timespec at the database layer

* use struct timespec { .tv_nsec = SAMBA_UTIME_OMIT } as special sentinel
  value in smbd when processing timestamps

Using NTTIME at the database layer is only done to avoid storing the special
struct timespec sentinel values on disk. Instead, with NTTIME the sentinel value
for an "unset" timestamp is just 0 on-disk.

The NTTIME value of 0 gets translated by nt_time_to_full_timespec() to the
struct timespec sentinel value { .tv_nsec = SAMBA_UTIME_OMIT }.

The function is_omit_timespec() can be used to check this.

Beside nt_time_to_full_timespec(), there are various other new time conversion
functions with *full* in their name that can be used to safely convert between
different types with the changed sentinel value.

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>