MFC r308160,r309194,r309216:

r308160 (by bapt):

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.

Relnotes: yes

r309194 (by bapt):

initialize *nextp which could be left uninitialized in case the configuration
file cannot be open/read

CID: 1365665

r309216 (by bapt):

Properly initialize nextp