rpc.mountd: let mountd consult /etc/services for port

At RHEL, if user set port for mountd at /etc/services as
"mount   12345/tcp", mountd should be bind to 12345, but the
latest nfs-utils, mountd get a rand port, not 12345.

This patch make sure mountd be bind to the port which was set
at /etc/service.

Signed-off-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
Signed-off-by: Steve Dickson <steved@redhat.com>