gpg: Make the internal sign_uids function easier to read.
* g10/keyedit.c (SIGN_UIDS_LOCAL): New. (SIGN_UIDS_NONREVOCABLE): New. (SIGN_UIDS_TRUSTSIG): New. (SIGN_UIDS_INTERACTIVE): New. (SIGN_UIDS_QUICK): New. (sign_uids): Replace several boolean args by flags arg.