Use a static callout to drive key_timehandler() instead of timeout().
While here, make key_timehandler() private to key.c.

Submitted by:	bz (2)
Tested by:	bz


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273557 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f