filedesc: avoid taking fdesc_mtx when not necessary in fddrop

No functional changes.
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