Enforce mail user and group
Explain how and where kern.cam.ada.write_cache can be set in practical
situations.

Reviewed by:	hrs
Approved by:	mav
Fix build with gcc
Match the correct variable to the variable description.

PR:		121173
Submitted by:	Thomas Mueller <tmueller at sysgo.com>
MFC after:	1 week
Lower warning level when built with gcc
Fix build on i386