syslogd: Log messages using libcasper

Some logging operations require access to external resources to
complete. Logging to F_WALL requires on-demand access to the user
accounting database. Logging to F_CONSOLE requires access to the
console. Logging to F_PIPE prompts execution of a command outside
of capability mode.

These operations cannot be performed in capability mode, so the
"p_open", "ttymsg", and "wallmsg" commands may be sent to libcasper to
circumvent these limitations.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41465