syslogd: Keep console and tty descriptors open

Console and tty descriptors are initially opened in parse_action() to
determine whether they are valid using isatty(). That descriptor is then
closed because it is never accessed by syslogd again; ttymsg() will reopen
the tty/console under a new descriptor when needed.

If the user attempts to log to a tty that is inactive outside of
syslogd, then syslogd must keep that descriptor open so the tty remains
accessible. For example, logging to /dev/ttyvb requires the initial
/dev/ttyvb descriptor to stay open so the user can view its buffer at
any time via CTRL+ALT+F12.

As a result, console and tty descriptors must remain open until a
potential configuration reload or a system shutdown. The given
descriptor will be closed in close_filed() in such circumstances.

PR: 274454
Fixes: c3175a6e1c5a ("syslogd: Do not open console descriptor")
Reported by: Trond Endrestøl <Trond.Endrestol@ximalas.info>
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D42215