gpgme_tool: Support GPG's new pinentry-mode.
* src/gpgme-tool.c (log_error): Do not always print the error source. (gt_set_pinentry_mode): New. (server_passphrase_cb): New. (cmd_pinentry_mode): New. (register_commands): Add cmd_pinentry_mode. (options): Add option --gpg-binary. (struct args): Add field gpg-binary. (parse_options, main): Implement that option.