Fixes ser2net not starting due to missing sysuser ser2net which is defined in the new upstream systemd unit file.

In the course of fixing that by adding the sysuser I also had to fix UUCP locking. Otherwise ser2net (now no longer running as root) was unable to create UUCP lockfiles when a user connected to a network proxied serial port.

The fix entails:

• requiring lockdev in libgensio (as this is where the locking happens)
• setting the UUCP lock path in libgensio to /run/lock/lockdev
• creating a sysuser for ser2net
• adding that user to the dialout group to allow access to serial devices
• adding that user to the lock group to allow access to the lock directory