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

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

Bump WARNS where possible.

Checked by: make universe
Approved by: ed (co-mentor)

This compiles cleanly with WARNS=5 on i386 and alpha, so try to make
sure that doesn't change.

Introduce the DEVFS "rule" subsystem. DEVFS rules permit the
administrator to define certain properties of new devfs nodes before
they become visible to the userland. Both static (e.g., /dev/speaker)
and dynamic (e.g., /dev/bpf*, some removable devices) nodes are
supported. Each DEVFS mount may have a different ruleset assigned to
it, permitting different policies to be implemented for things like
jails.

Approved by: phk