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

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

Link g_label_disk_ident when building geom_label as a module

It seems that it is preferable to keep support for glabel also for
filesystems that we don't support natively.
Revert part of r241636 to do so.

This patch is not targeted for MFC.

Requested by: gleb, jhb

Disconnect non-MPSAFE NTFS from the build in preparation for dropping
GIANT from VFS. This code is particulary broken and fragile and other
in-kernel implementations around, found in other operating systems,
don't really seem clean and solid enough to be imported at all.
If someone wants to reconsider in-kernel NTFS implementation for
inclusion again, a fair effort for completely fixing and cleaning it
up is expected.

In the while NTFS regular users can use FUSE interface and ntfs-3g
port to work with their NTFS partitions.

This is not targeted for MFC.

Add missing reference to GPT support.

Submitted by: Paul B. Mahol onemda at gmail.com
Approved by: re (kib)

Add NTFS labeling function.

Reviewed by:pjd

Add code for Ext2FS and ReiserFS labels recognition.

Submitted by: Stanislav Sedov <stas@310.ru>
PR: kern/84638
MFC after: 1 week

Introduce GEOM_LABEL class.
This class is used for detecting volume labels on file systems:
UFS, MSDOSFS (FAT12, FAT16, FAT32) and ISO9660.
It also provide native labelization (there is no need for file system).

g_label_ufs.c is based on geom_vol_ffs from Gordon Tetlow.
g_label_msdos.c and g_label_iso9660.c are probably hacks, I just found
where volume labels are stored and I use those offsets here,
but with this class it should be easy to do it as it should be done by
someone who know how.
Implementing volume labels detection for other file systems also should
be trivial.

New providers are created in those directories:
/dev/ufs/ (UFS1, UFS2)
/dev/msdosfs/ (FAT12, FAT16, FAT32)
/dev/iso9660/ (ISO9660)
/dev/label/ (native labels, configured with glabel(8))

Manual page cleanups and some comments inside were submitted by
Simon L. Nielsen, who was, as always, very helpful. Thanks!