Pull in upstream libcxxrt commit 8006101, which makes its cxxabi.h file
compilable just by itself.

PR:		kern/184019
MFC after:	3 days
Remove unused defines.
Add some context for the "kldload sem" command; minor other improvements.

PR:		183650
Submitted by:	Bjorn Heidotting
MFC after:	1 week
Add cross-references to casperd(8) and libcapsicum(3).

Suggested by:	rwatson