Fix two issues with lockmgr(9) LK_CAN_SHARE() test, which determines
whether the shared request for already shared-locked lock could be
granted. Both problems result in the exclusive locker starvation.

The concurrent exclusive request is indicated by either
LK_EXCLUSIVE_WAITERS or LK_EXCLUSIVE_SPINNERS flags. The reverse
condition, i.e. no exclusive waiters, must check that both flags are
cleared.

Add a flag LK_NODDLKTREAT for shared lock request to indicate that
current thread guarantees that it does not own the lock in shared
mode. This turns back the exclusive lock starvation avoidance code;
see man page update for detailed description.

Use LK_NODDLKTREAT when doing lookup(9).

Reported and tested by: pho
No objections from: attilio
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks