rpc: clean up decoding of gssproxy linux creds

We can use the normal coding infrastructure here.

Two minor behavior changes:

	- we're assuming no wasted space at the end of the linux cred.
	  That seems to match gss-proxy's behavior, and I can't see why
	  it would need to do differently in the future.

	- NGROUPS_MAX check added: note groups_alloc doesn't do this,
	  this is the caller's responsibility.

Tested-by: Simo Sorce <simo@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>