unmount and unregister v3 on startup based on config params

Currently, v2 to v4 ports are being unregistered and unmounted
when Ganesha is configured in "nfsv4 only" mode. So add a check for
Ganesha config params to only unregister all ports when Ganesha
is accepting v3 requests; otherwise unregister v4 port only.

Signed-off-by: Viji Ganapathi <vganapathi@panasas.com>