agent: Fix function return type for check_cb and agent_askpin.
* agent/call-pinentry.c (unlock_pinentry): Return gpg_error_t. (start_pinentry, setup_qualitybar): Likewise. (agent_askpin): Fix return value check of check_cb. * agent/command-ssh.c (reenter_compare_cb): Return gpg_error_t. (ssh_identity_register): Fix return value check of agent_askpin. * agent/cvt-openpgp.c (try_do_unprotect_cb): Return gpg_error_t. * agent/findkey.c (try_unprotect_cb): Likewise. * agent/genkey.c (reenter_compare_cb): Return gpg_error_t. (agent_ask_new_passphrase): Fix return value check of agent_askpin.