Various gnupload improvements.

Various gnupload improvements.

Add support for uploading to download.gnu.org.ua.
Add support for a .gnupload configuration file.
Support creating and removing symlinks.
Allow for several operations in a single invocation.
Add debugging features.

* lib/gnupload: New options --delete, --symlink, --rmsymlink,
--symlink-regex, --dry-run; support `--' to separate options and
commands from files.  New target download.gnu.org.ua.  Expand
`.gnupload' file contents before command line arguments.
(usage): Expand.
(dprint, mkdirective, mksymlink, upload): New functions.
* THANKS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>