Return a value indicating whether FILENAME is locked.
The value is nil if the FILENAME is not locked,
t if it is locked by you, else a string saying which user has locked it.

Lock FILE, if current buffer is modified.
FILE defaults to current buffer's visited file,
or else nothing is done if current buffer isn't visiting a file.

Unlock the file visited in the current buffer.
If the buffer is not modified, this does nothing because the file
should not be locked in that case.