errno not be set on RPC errors

errno not be set on RPC errors

Changed both nfs_advise_umount() and nfs_gp_ping() to
set the errno by calling CLNT_GETERR() after a CLNT_CALL()
error. Also added code to rpc_strerror() that will log
the errno value, when set, via strerror().

These changes added essential information to the error message
making it much easier to detect errorsuch as "Connection refused"

Signed-off-by: Steve Dickson <steved@redhat.com>