libalias: fix subtle racy problem in outside-inside forwarding

sys/netinet/libalias/alias_db.c has internal static function UseLink()
that passes a link to CleanupLink() to verify if the link has expired.
If so, UseLink() may return NULL.

_FindLinkIn()'s usage of UseLink() is not quite correct.

Assume there is "redirect_port udp" configured to forward incoming
traffic for specific port to some internal address.
Such a rule creates partially specified permanent link.

After first such packet libalias creates new fully specifiled
temporary LINK_UDP with default timeout 60 seconds.
Also, in case of low traffic libalias may assign "timestamp"
for this new temporary link way in the past because
LibAliasTime is updated seldom and can keep old value
for tens of seconds, and it will be used for the temporary link.

It may happen that next incoming packet for redirected port
passed to _FindLinkIn() results in a call to UseLink()
that returns NULL due to detected expiration.
Immediate return of NULL results in broken translation:
either a packet is dropped (deny_incoming mode) or delivered to
original destination address instead of internal one.

Fix it with additional check for NULL to proceed with a search
for original partially specified link. In case of UDP,
it also recreates temporary fully specified link
with a call to ReLink().

Practical examples are "redirect_port udp" rules for unidirectional
SYSLOG protocol (port 514) or some low volume VPN encapsulated in UDP.

Thanks to Peter Much for initial analysis and first version of a patch.

Reported by: Peter Much <pmc@citylink.dinoex.sub.org>
PR: 269770
MFC after: 1 week