Support RELP receive on logserver.

The Reliable Event Logging Protocol is a protocol specially designed
to not lose log messages.  Rsyslog implements it (actually invented
it).  While NSC currently does not have any particular needs for more
reliable delivery than normal syslog-over-TCP, or even UDP, it is
easy enough to enable in the rsyslog daemon, and we already have
support for RELP in the syslog::client class.

RELP does not have a standardized port number, but several examples
in the rsyslog documentation use 2514, and we used that as the default
in syslog::client, so let's continue with that.  (The omrelp plugin
for *sending* logs over RELP actually defaults to use port 514, but
that is not a very good choice, since that is commonly used for normal
syslog over TCP, including by us.)