Factor out ulog serial number status check

Factor out ulog serial number status check

Add a new function ulog_get_sno_status, which checks a serial number
and timestamp against the ulog for currency.  Use it in kdb5_util dump
and in ulog_get_entries.  Adjust parse_iprop_header's contract in
dump.c to better match the ulog_get_sno_status contract.

This change causes some minor behavior differences.  kadmind will
check for an empty ulog unless the last serial number matches exactly,
and will never set lastentry when returning UPDATE_FULL_RESYNC_NEEDED
(which was pointless).  kdb5_util dump will recognize a dump file as
current if it exactly matches the last serial number, even if the ulog
is empty; it will be more robust in the presence of non-monotonic
clocks; and it will properly lock around the ulog access.