gpg: Fix algo string parsing of --quick-addkey.

* g10/keygen.c (parse_key_parameter_string): Fix handling of PART==1.
(parse_key_parameter_part): Use default key size if only "rsa", "dsa",
or "elg" is given.

The first change is the actual fix. The second change avoids the
error "Invalid Curve" when only "rsa" instead of RSA2048 is given.

• Fixes-commit: ce29272e24e7b718b8fca9b84bc728e65f3dea24
• Signed-off-by: Werner Koch <wk@gnupg.org>