[MINOR] checks: add PostgreSQL health check

I have written a small patch to enable a correct PostgreSQL health check
It works similar to mysql-check with the very same parameters.

E.g.:
listen pgsql 127.0.0.1:5432
   mode tcp
   option pgsql-check user pgsql
   server masterdb pgsql.server.com:5432 check inter 10000
4 files changed