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.
abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'