Add locking to protect against racy updates from multiple processes.
Add locking to protect against racy updates from multiple processes.

When opening the fsfile (whether directly or via a socket to a
persistent background process), the opening process attempts to create
a symlink whose contents (rather than pointing to another file by
name) provide some basic info (username, hostname, and PID) of the
process holding the lock.

zevweiss/jemf,zevweiss/jemf