New port: net-mgmt/mysql-snmp: Net-SNMP Perl agent for monitoring MySQL servers

mysql-snmp is a MySQL Net-SNMP agent written in Perl, and using the
Net-SNMP Perl bindings.

It connects to a MySQL server and returns information to Net-SNMP when
needed. It parses SHOW INNODB STATUS and other MySQL status variables.

WWW: https://github.com/masterzen/mysql-snmp

Reported by: mwlucas
Reviewed by: danfe
Differential Revision: https://reviews.freebsd.org/D22480