Fix many issues related to mount/unmount:

Fix many issues related to mount/unmount:

1. Root from inside a jail was able to unmount any file system
   (except /).
2. Unprivileged root was able to unmount file systems mounted by
   privileged root (execpt /).
3. User from inside a jail was able to mount file system when
   sysctl vfs.usermount was set to 1.
4. User was able to mount file system when vfs.usermount was set to 1
   (that's ok) and unmount it even if vfs.usermount was equal to 0
   (that's not correct).

Possibility from point 1 was reported by: Dariusz Kowalski <darek@76.pl>

Only a part of this fix will be MFC'ed (if approved).

PR:		kern/60149
Reviewed by:	rwatson
Approved by:	scottl (mentor)
MFC after:	3 days