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

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

MFC r230377:

Don't print the nmount(2) provided error message if it is empty.

MFC r230373:

Change mount_fs() to not exit on error. The "failok" mount option
requires that errors are passed to the caller.

PR: 163668

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

Approved by: re (implicit)

Remove the advertising clause from UCB copyrighted files in sbin. This
is in accordance with the information at
ftp://ftp.cs.berkeley.edu/pub/4bsd/README.Impt.License.Change

Be a little bit more pestimistic in argument handling - check if we've
overflown our internal buffer (though after the fact), and s/strncpy/strlcpy/

Reviewed by: rodrigc
Obtained from: Juniper Networks

Pass a char buffer parameter with name "errmsg" to nmount().
For filesystems which use vfs_mount_error() to log an error, this
char buffer will be populated with a string error message.
If nmount() fails, in addition to printing out strerror(errno),
print out the "errmsg" populated by vfs_mount_error().

- Make size parameter to build_iovec() a size_t, not an int
- Add build_iovec_argf() helper function, for help converting old
mount options which used the mount_argf() function for the mount() syscall.

Discussed with: phk

- Minor fixes to raise WARNS level to 6.
- Teach the mount program to call the nmount() syscall directly
- Preserve existing method of calling mount() for UFS, until we clean things
up.
- Preserve existing method of forking and calling external mount programs for
mfs, msdosfs, nfs, nfs4, ntfs, nwfs, nullfs, portalfs, reiserfs, smbfs,
udf, umapfs, unionfs
- devfs, linprocfs, procfs, ext2fs call nmount() syscall directly, since
that is all those external mount programs were doing

Reviewed by: phk
Discussed on: arch