* tests/openpgp/gpgtar.test: Test signing.
* tools/gpgtar-create.c (gpgtar_create): Add 'sign' option, add the
appropriate gpg arguments to implement signing and selecting the local
user.
* tools/gpgtar.c (parse_options): We do handle '--local-user' now.
(main): Handle signing, encrypting, and doing both when creating an
archive.
* tools/gpgtar.h (gpgtar_create): Update prototype.