detect stale lock file

[ukleinek]

When a microcom process died but failed to remove the lock file (/var/lock/LCK..$devname), restarting microcom fails with

lockfile for port exists

This could be improved by checking the content of the file and ignore its presence if the related pid doesn't exist.

[ukleinek]

Since 34e7fe5 lockfiles are not used any more, so the problem is gone.