syslogd(8): add an 'include' keyword

All the '.conf' files not beginning with a '.' contained int he directory
following the keyword will be included.

This keyword can only be used in the first level configuration files.

Modify the default syslogd.conf to 'include' /etc/syslog.d and
/usr/local/etc/syslog.d

It simplify a lot handling of syslog from automation tools.

Reviewed by: markj, kib (via irc)
Approved by: markj
MFC after: 2 weeks
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D8402