File lock on read only systems

[szyg]

On read-only file systems the FileLock attempts to create 'serial.lock' in the current folder which fails. On linux systems it would make more sense to default to /var/lock or /tmp which are usually mounted as tmpfs on read-only systems. It's simple enough to manually modify line 292 of notecard.py but it would be nice if it defaulted to /var/lock or was overridable in the init to a file of choice.