Add symlink method to util.opener.
Differences from os.symlink:
- the symlink name is relative to the opener base directory
- if a file with that name already exists, it's removed
- if necessary, parent directories are created
- if the system (OS or filesystem) doesn't support symlinks, a
regular file is created. Its contents are the symlink target.
adding da/foo
adding foo
% manifest
200 Script output follows
drwxr-xr-x da
-rw-r--r-- 4 foo
200 Script output follows
-rw-r--r-- 4 foo