MFC: r260889, r260890, r260911:

MFC: r260889, r260890, r260911:

	r260911 | imp | 2014-01-20 10:45:36 -0700 (Mon, 20 Jan 2014) | 5 lines

	Don't lock in the generic grab just to lock again in the specific grabs.

	r260890 | imp | 2014-01-19 12:39:13 -0700 (Sun, 19 Jan 2014) | 11 lines

	Introduce grab and ungrab upcalls. When the kernel desires to grab the
	console, it calls the grab functions. These functions should turn off
	the RX interrupts, and any others that interfere. This makes mountroot
	prompt work again. If there's more generalized need other than
	prompting, many of these routines should be expanded to do those new
	things.

	Reviewed by:    bde (with reservations)

git-svn-id: svn://svn.freebsd.org/base/stable/10@262649 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f