Introduce and use new flag -L to mount for mounting only late filesystems.

Previously, rc.d/mountlate mounted *all* filesystems, causing problems with
background NFS mounts being mounted twice.

PR:		conf/137629
Submitted by:	eadler (original concept)
Reviewed by:	mjg
Approved by:	hrs
Completely rewrite the interface to smbdev switching from dev_clone
to cdevpriv(9). This commit changes the semantic of mount_smbfs
in userland as well, which now passes file descriptor in order to
to mount a specific filesystem istance.

Reviewed by:	attilio, ed
Tested by:	martymac
Overhaul locking in netsmb, getting rid of the obsolete lockmgr() primitive.
This solves a long standing LOR between smb_conn and smb_vc.

Tested by:	martymac, pho (previous version)