sed: Bounds check the file path used in the 'w' command.

Modified version of a diff from Sebastien Marie to prevent a crash found
with the afl fuzzer.

Obtained from:	OpenBSD (CVS Rev. 1.37)
MFC after:	1 week
Add an UPDATING entry and warning about the change in r274807 to help users
transition to the new behavior.

Discussed with:	jmallett
Sponsored by:	DARPA, AFRL
Do not count RCTD bit set as an error.

We can not really implement it, but specification tells that it "shall"
work, so it can be safely ignored.

MFC after:	1 week
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Initialize an argument to NULL instead of expecting dlinfo() to do it.

dlinfo() is a weak reference that may not be initialized at the time of
execution. The default implementation (in lib/libc/gen/dlfcn.c) neither
modifies the address pointed to by the third argument nor returns an error.

Differential Revision:	https://reviews.freebsd.org/D1326
Reviewed by:		markj
MFC after:		1 week
Add some additional clarification and fix a few gammer nits.

Reviewed by: kib
MFC after:   3 weeks
Bump Dd for r275846

MFC after:	3 weeks
Fix 8259 IRQ priority resolver.
Initialize the 8259 such that IRQ7 is the lowest priority.

Reviewed by:		tychon
Differential Revision:	https://reviews.freebsd.org/D1322
MFC after:		1 week
Merge remote-tracking branch 'freebsd/master' into hardened/current/master