Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

- Add vnode-backed swap space specification support. This is enabled when
device names "md" or "md[0-9]*" and a "file" option are specified in
/etc/fstab like this:

md none swap sw,file=/swap.bin 0 0

- Add GBDE/GELI encrypted swap space specification support, which
rc.d/encswap supported. The /etc/fstab lines are like the following:

/dev/ada1p1.bde none swap sw 0 0
/dev/ada1p2.eli none swap sw 0 0

.eli devices accepts aalgo, ealgo, keylen, and sectorsize as options.

swapctl(8) can understand an encrypted device in the command line
like this:

# swapctl -a /dev/ada2p1.bde

- "-L" flag is added to support "late" option to defer swapon until
rc.d/mountlate runs.

- rc.d script change:

rc.d/encswap -> removed
rc.d/addswap -> just display a warning message if $swapfile is defined
rc.d/swap1 -> renamed to rc.d/swap
rc.d/swaplate -> newly added to support "late" option

These changes alleviate a race condition between device creation/removal
and swapon/swapoff.

MFC after: 1 week
Reviewed by: wblock (manual page)