lib: Added i_unlink() and i_unlink_if_exists()

lib: Added i_unlink() and i_unlink_if_exists()
These log the error message on a failed unlink(). They also include the
source code file and line number to make it easier to find which unlink()
actually failed if the path itself doesn't already clearly identify it.
This can be especially useful if the path is (null), "" or contains some
corrupted garbage.