Rework ".." lookup; previous one failed to properly busy the mountpoint.

Reviewed by:	kib@
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Add description for the "automounted" mount flag.

Reviewed by:	emaste@
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Remove useless - and buggy, it resulted in spurious warnings in logs - code.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation