libthr: Fix try lock

[aokblast]

Summary

1. When contester > 2, a release causes kernel set CONTENTION bit but with no owner. In this case, the userspace should be able to try the lock instead of fall immediately. We copy this from trylock2.
2. A PI mutex should consult to kernel instead of decided by userspace itself.

MFC after: 2 weeks

[kib]

Besides the formatting bug, I think that the algorithmic change is fine.

lib/libthr/thread/thr_umtx.h

87

Fix indent, all continuation lines for the same statement must use the same indent. Also fill the previous line more.

[aokblast]

Reflect kib@'s comment