thread: add td_wantedlock

This enables obtaining lock information threads are actively waiting for
while sampling. Without the change one would only see a bunch of calls
to lock_delay(), where the stacktrace often does not reveal what the
lock might be.

Note this is not the same as lock profiling, which only produces data
for cases which wait for locks.

struct thread already has a td_lockname field, but I did not use it
because it has different semantics -- denotes when the thread is off
cpu. At the same time it could not be converted to hold a lock_object
pointer because non-curthread access would no longer be guaranteed to be
safe -- by the time it reads the pointer the lock might have been taken,
released and the object containing it freed.

Sample usage with dtrace:
rm /tmp/out.kern_stacks ; dtrace -x stackframes=100 -n 'profile-997 { @[curthread->td_wantedlock != NULL ? stringof(curthread->td_wantedlock->lo_name) : stringof("\n"), stack()] = count(); }' -o /tmp/out.kern_stacks

This also facilitates addition of lock information to traces produced by
hwpmc.

Note: spinlocks are not supported at the moment.

Sponsored by: Rubicon Communications, LLC ("Netgate")