libexport.a: Refactor netgroup checking in client_check()

libexport.a: Refactor netgroup checking in client_check()

Clean up: refactor netgroup logic out of client_check() to make it
easier to introduce IPv6 support.

 +  Use preferred style of keeping #ifdef out of the middle of
    function definitions.  Squelch compiler warnings for "#ifndef
    HAVE_INNETGR" by using __attribute__((unused)).

 +  Use preferred style of not using curly braces around switch cases.

 +  Match style used for check_{fqdn,subnetwork}.

 +  Clarify comment documenting use of h_aliases

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