Add support for property based filtering of messages

[troglobit]

When setting up syslogd to act as a remote server for other devices on a network you often want to group logs from a sender to a separate log file.

Similarly it would be interesting to be able to collect all logs from a network based on a RFC5424 property, like msgid, or locally sort logs based on daemon name, since most services use the LOG_DAEMON facility.

FreeBSD and NetBSD seems to have added support for this, the former seem to be more complete:

• https://man.freebsd.org/cgi/man.cgi?query=syslog.conf&sektion=5&n=1#PROPERTY-BASED_FILTERS
• https://man.netbsd.org/syslog.conf.5