Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Provide a set of sysctls and tunables to disable device node creation
for specific "kinds" of disk labels - for example, GPT UUIDs. Reason
for this is that sometimes, other GEOM classes attach to these device
nodes instead of the proper ones - e.g. they attach to /dev/gptid/XXX
instead of /dev/ada0p2, which is annoying.

Reviewed by: pjd (earlier version)
MFC after: 1 month

- Use the correct argument when determining the buffer size.

PR: kern/131575
MFC after: 2 days

Remove trailing spaces.

Handle MSDOS file systems properly. Before the change file systems
created on Windows XP (and others maybe) were not detected.
We detected only those created with newfs_msdos(8).

Submitted by: Tobias Reifenberger <treif@mayn.de>
style(9)ified by: pjd

Remove trailing spaces.

Don't pass error value pointer to g_read_data(9) at all if we don't
have any use of it.

Suggested by: pjd

Check for g_read_data(9) errors properly:

o The only indication of error condition is NULL value returned by
the function;

o value pointed to by error argument is undefined in the case when
operation completes successfully.

Discussed with: phk

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!