Output strerror from xo_warn

Reported by: bapt
Reviewed by: bapt, ngie

Differential Revision: https://reviews.freebsd.org/D1378
Support ALT_BREAK_TO_DEBUGGER in vt(4)

Submitted by:	Andre Albsmeier on -hackers
Merge remote-tracking branch 'freebsd/master' into hardened/current/master