Extend struct ucred with group table.

This saves one malloc + free with typical cases and better utilizes
memory.

Submitted by:	Tiwei Bie <btw mail.ustc.edu.cn> (slightly modified)
X-Additional:	JuniorJobs project


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274122 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f