MFC r283495:

Hide vfs.pfs.trace variable if it is not used.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Remove opt_mac.h generation for various kernel modules that no longer
require it.

Submitted by: pjd

Add PSEUDOFS_TRACE option.

Introduce support for Mandatory Access Control and extensible
kernel access control.

Modify pseudofs so that it can support synthetic file systems with
the multilabel flag set. In particular, implement vop_refreshlabel()
as pn_refreshlabel(). Implement pfs_refreshlabel() to invoke this,
and have it fall back to the mount label if the file system does
not implement pn_refreshlabel() for the node. Otherwise, permit
the file system to determine how the service is provided.

Approved by: des
Obtained from: TrustedBSD Project
Sponsored by: DARPA, NAI Labs

Move the pseudofs, procfs and linprocfs modules out from the fs directory.
Keeping them there seemed like a good idea at the time, but it annoys bde
and confuses people who do not understand how MODULES_OVERRIDE works.