* Instead of an error string, return the usual 0 or -1 and let the caller
  figure out what, if anything, to tell the user.

* Avoid string manipulations by opening /dev first and using openat()
  with O_RESOLVE_BENEATH.

* Add a boolean argument which, if false, causes ttymsg() to return
  without sending the message if the tty's group-writable bit is not
  set.  This saves programs that respect this setting (like syslogd(8))
  from having to check before calling ttymsg().

* Update all callers.

The observable effect of this change is minimal except for slightly
different error messages when ttymsg() fails.  However, syslogd(8) will
no longer print spurious error messages on the console after trying and
failing to write a log message to an X11 session.

PR:		295171
MFC after:	1 week
Reviewed by:	jfree, markj
Differential Revision:	https://reviews.freebsd.org/D57018

(cherry picked from commit 05e8f2bf0906875e666469e0338f922d1113d034)

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

- Move all of the code responsible for transmitting log messages into a
  separate function, fprintlog_write().
- Instead of manually modifying a list of iovecs, add a structure
  iovlist with some helper functions.
- Alter the F_FORW (UDP message forwarding) case to also use iovecs like
  the other cases. Use sendmsg() instead of sendto().
- In the case of F_FORW, truncate the message to a size dependent on the
  address family (AF_INET, AF_INET6), as proposed by RFC 5426.
- Move all traditional message formatting into fprintlog_bsd(). Get rid
  of some of the string copying and snprintf()'ing. Simply emit more
  iovecs to get the job done.
- Increase ttymsg()'s limit of 7 iovecs to 32. Add a definition for this
  limit, so it can be reused by iovlist.
- Add fprintlog_rfc5424() to emit RFC 5424 formatted log entries.
- Add a "-O" command line option to enable RFC 5424 formatting. It would
  have been nicer if we supported "-o rfc5424", just like on NetBSD.
  Unfortunately, the "-o" flag is already used for a different purpose
  on FreeBSD.
- Don't truncate hostnames in the RFC 5424 case, as suggested by that
  specific RFC.

For people interested in using this, this feature can be enabled by
adding the following line to /etc/rc.conf:

  syslogd_flags="-s -O rfc5424"

Differential Revision:	https://reviews.freebsd.org/D15011

  also use this, and they shouldn't have to have their own prototypes.

- Silence warnings about constness and signedness in ttymsg().  This
  includes changing the return value to a `const char *', and changing
  the types of `left' and `wret' (both byte counts) to ssize_t.

Reviewed by:	bde