[syslog] Allow port number to be specified for encrypted syslog server

[syslog] Allow port number to be specified for encrypted syslog server

The original implementation in commit 943b300 ("[syslog] Add basic
support for encrypted syslog via TLS") was based on examples found in
the rsyslog documentation rather than on RFC 5425, and unfortunately
used the default syslog port number 514 rather than the syslog-tls
port number 6514 defined in the RFC.

Extend parsing of the syslog server name to allow for an optional port
number (in the relatively intuitive format "server[:port]").  Retain
the existing (and incorrect) default port number to avoid breaking
backwards compatibility with existing setups.

Reported-by: Christian Nilsson <nikize@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>