client: use a dedicated file lock to prevent concurrent changes

client: use a dedicated file lock to prevent concurrent changes

We were using a lock on the Unix socket. This was working on Linux but
this is not portable. Therefore, we have to use a dedicated file for
this purpose. We use /var/lock by default.

We don't do a secure creation as the lock file is only opened in
append mode, so a symlink attack could only create empty file or reset
the timestamp of a file. No content can be erased this way.

Fix #445