Add a testing facility to manually reclaim a vnode

Add the debug.try_reclaim_vnode sysctl. When a pathname is written to it, it
will be reclaimed, as long as it isn't already or doomed. The purpose is to
gain test coverage for vnode reclamation, which is otherwise hard to
achieve.

Add the debug.ftry_reclaim_vnode sysctl. It does the same thing, except
that its argument is a file descriptor instead of a pathname.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20519