loop.

When syslogd forks a process for '|' destinations, it closes all file
descriptors greater than 2.

Use closefrom() for this instead of a getdtablesize()/close() loop because
it is both faster and avoids leaving file descriptors open because the limit
was lowered after they were opened.

- Make sure that errno isn't modified before calling logerror() in error
  conditions.
- Don't check for AF_INET6 when compiled without INET6 support.

Approved by:	emaste (co-mentor)

- 	Add support for ipv6 addresses as destination
- 	Add documentation for IPv6 support

PR:		docs/171580
Approved by:	cperciva (implicit)

Update man page to present -T argument in synopsis

behavior, which binds to the well known UDP port.

This option implies -s.

MFC after:	2 months

Checking if it is > 0 doesn't make sense, because snprintf returns
how much space is needed if the buffer is too small. Instead, check
if the return value was greater than the buffer size, and truncate
the message if it was too long.

It isn't clear if snprintf can return a negative value in the case
of an error - I don't believe it can. If it can, then testing
v->iov_len won't help 'cos it is a size_t, not an ssize_t.

Also, as clang points out, we must always increment v here, because
later code depends on the message being in iov[5].

WARNS?=3 until that's resolved.

WARNS=6 setting in HEAD (unlike stable/8), so it's best
to remove the line entirely.

Pointed out by:	jh
Approved by:	des (mentor)

standard ports, but it can't *receive* them (port 514 is
hardcoded).  This commit adds that missing feature.

(NB:  I actually needed this feature for a server farm where
multiple jails run with shared IP addresses, and every jail
should have its own syslogd process.)

As a side effect, syslogd now compiles with WARNS=6.

Approved by:	des (mentor)
MFC after:	3 weeks