charon-cmd: Add --agent option to authenticate using ssh-agent(1)

The socket path is read from the SSH_AUTH_SOCK environment variable.
So using this with sudo might require the -E command line (or an appropriate
sudoers config) to preserve the environment.

src/charon-cmd/cmd/cmd_options.h

@@ -35,6 +35,7 @@ enum cmd_option_type_t {
 	CMD_OPT_REMOTE_IDENTITY,
 	CMD_OPT_CERT,
 	CMD_OPT_RSA,
+	CMD_OPT_AGENT,
 	CMD_OPT_LOCAL_TS,
 	CMD_OPT_REMOTE_TS,
 	CMD_OPT_PROFILE,