MFC r197711 (partial) to 6.x and 7.x:

MFC r197711 (partial) to 6.x and 7.x:

- Add no zero mapping feature, disabled by default. [EN-09:05]

MFC 178913,178914,179242,179243,180336,180340 to 6.x:

- Fix kqueue pipe race conditions. [SA-09:13]

MFC r192301 to 7.x; 6.x has slightly different fix:

- Fix devfs / VFS NULL pointer race condition. [SA-09:14]

Security:	FreeBSD-SA-09:13.pipe
Security:	FreeBSD-SA-09:14.devfs
Errata:		FreeBSD-EN-09:05.null
Submitted by:	kib [SA-09:13] [SA-09:14]
Submitted by:	bz [EN-09:05]
In collaboration with:	jhb, kib, alc [EN-09:05]
Approved by:	so (simon)

git-svn-id: svn://svn.freebsd.org/base/releng/7.2@197715 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f