dreln: handle target string that is exactly old path

If there is a link target that exactly matches the oldpath, then this link is not currently updated. For example:

mylink1 --> /old/path
mylink2 --> /old/path/foo

Then after running the command:

dreln /old/path /new/path ...

One would like for:

mylink1 --> /new/path
mylink2 --> /new/path/foo

But one instead gets:

mylink1 --> /old/path
mylink2 --> /new/path/foo

One could detect that with MFU_PATH_EQUAL. However, the mfu_path_slice() call will then attempt to slice all components, which ends up wrapping back to the front and slicing nothing, so that the new path is prepended to the old path in the target.

As a fix, we should also update mfu_path_slice to avoid wrapping back. We could redefine the functionality here to slice all components if the offset is beyond the length of the path. In that case, we should also update other functions like mfu_path_cut to behave similarly.

Start by tracing dreln's target update logic and the mfu_path_slice() call for a target exactly matching oldpath. Reproduce the example with /old/path and /new/path, then verify that the exact target is updated without path wrapping; also inspect mfu_path_cut() for the requested consistent behavior.