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
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/segvguard
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/log
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/userlandenhanced
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/libressl
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/segvguard' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/log' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/userlandenhanced' into hardened/current/unstable
Merge remote-tracking branch 'origin/hardened/current/unstable' into hardened/current/unstable-with-libressl
Merge remote-tracking branch 'origin/hardened/current/libressl' into hardened/current/unstable-with-libressl