Improve pid locking code to work on filesystems that don't support hard links.

Improve pid locking code to work on filesystems that don't support hard links.

Probing for hard link support in the pid locking code is redundant since
git-annex init already probes that. But, it didn't seem worth threading
that data through; the pid locking code runs at most once per git-annex
process, and only on unusual filesystems. Optimising a single hard link
and unlink isn't worth it.

This commit was sponsored by Francois Marier on Patreon.