syslogd: Add host forwarding test

Test communication between hosts by setting up two syslogd instances,
each binded to their own addresses, and sending a message from one to
the other. The tests passes if the message was delivered and logged
successfully.

This required some tweaking to the syslogd start and stop routines so
they could handle launching multiple syslogd instances with different
runtime files.

Run the tests in jails with an isolated VNET so they don't fail due to
an address bind collision in the host's VNET.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D47649