Add sysctl kern.proc.cwd

It returns only current working directory of given process which saves a lot of
overhead over kern.proc.filedesc if given proc has a lot of open fds.

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@274167 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f