wks: Add new commands --install-key and --remove-key to the client.

* tools/gpg-wks-client.c (aInstallKey, aRemoveKey, oDirectory): New.
(opts): Add "--install-key", "--remove-key" and "-C".
(parse_arguments): Parse them.
(main): Check that the given directory exists.  Implement the new
commands.

These commands maybe useful to prepare a WKD directory on a non-Unix
box using the standard wks client.

(cherry picked from commit 602b1909632925d5a2e0778c102d66109795c627)

• Signed-off-by: Werner Koch <wk@gnupg.org>