Replaced OS-independent lock with own set.

If the server crashes, any file locks are discarded.
Otherwise, locked files may never be deleted etc.
Added file lock during upload-process.

Files that are uploaded, cannot be modified.
Upload messages are displayed now.
Corrected ERRORCODE_502
Implemented full locking functionality.

Not multi-user-safe yet.
Implemented two-side EOM.

Now the client also sends EOMs which the server ignores.
Deleted TODO task for EOM.