mingw: implement basic `symlink()` functionality (file symlinks only)

mingw: implement basic `symlink()` functionality (file symlinks only)

Implement `symlink()`. This implementation always creates _file_
symlinks (remember: Windows discerns between symlinks pointing to
directories and those pointing to files). Support for directory symlinks
will be added in a subseqeuent commit.

This implementation fails with `ENOSYS` if symlinks are disabled or
unsupported.

Signed-off-by: Karsten Blees <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>