Document options for .eli devices.

Submitted by:	ADAM David Alan Martin
Fix compiler warnings.
Introduce new structure sfstat for collecting sendfile's statistics
and remove corresponding fields from struct mbstat. Use PCPU counters
and SFSTAT_INC() macro for update these statistics.

Discussed with:	glebius
MFi386: add ddb "show sysregs" command.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Move the Raspberry Pi low level options out of the kernel config and into
std.rpi + std.bcm2835.

Reviewed by:	imp
When stopping pfsync, remove the pfsync interface's syncpeer setting.

MFC after:	3 days
Nuke "systat -mbuf". It was broken since FreeBSD 5, and since there
haven't been any complaints, no one used it.
Nuke mbstat. It wasn't used for mbuf statistics since FreeBSD 5.

Now that r253351 moved sendfile() stats to a separate struct, the
last field used in mbstat is m_mcfail, which is updated, but never
read or obtained from userland.