Add client-side rpc_gss_*() APIs

Add client-side rpc_gss_*() APIs

Introduce new client-side RPCSEC API functions that match the same
libtirpc API in FreeBSD and Solaris. This includes
rpc_gss_seccreate(3t), rpc_gss_set_defaults(3t), and
rpc_gss_max_data_length(3t).

The man pages, written by Doug Rabson, come from FreeBSD, with
some adjustments by me.

The new code was written from scratch based on FreeBSD's
implementation, but adapted to invoke the existing legacy U-M APIs
in our implementation. We will continue to provide the legaacy APIs
until API consumers are switched to the new ones. FreeBSD never had
the legacy U-M GSS APIs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>