If stdin is not a terminal pass-through its data to the session

This allows to send commands to a running session:

    echo cowsay hi | abduco -a session

See #8 for initial discussion of the feature.

abduco.1

@@ -76,6 +76,9 @@ If for some reason the
 domain socket representing a session is deleted, sending
 .BR SIGUSR1
 to the server process will recreate it.
+
+If standard input does not refer to a terminal, its content is passed
+through to the underlying session.
 .SH OPTIONS
 If no command line arguments are given all currently active sessions are
 printed sorted by their respective creation date. Lines starting with an

abduco.c

@@ -120,7 +120,7 @@ typedef struct {
 static Server server = { .running = true, .exit_status = -1, .host = "@localhost" };
 static Client client;
 static struct termios orig_term, cur_term;
-static bool has_term, alternate_buffer, quiet;
+static bool has_term, alternate_buffer, quiet, passthrough;
 
 static struct sockaddr_un sockaddr = {
 	.sun_family = AF_UNIX,
@@ -588,6 +588,8 @@ int main(int argc, char *argv[]) {
 	server.name = basename(argv[0]);
 	gethostname(server.host+1, sizeof(server.host) - 1);
 
+	passthrough = !isatty(STDIN_FILENO);
+
 	while ((opt = getopt(argc, argv, "aAclne:fqrv")) != -1) {
 		switch (opt) {
 		case 'a':
@@ -623,6 +625,11 @@ int main(int argc, char *argv[]) {
 		}
 	}
 
+	if (passthrough) {
+		quiet = true;
+		client.flags |= CLIENT_LOWPRIORITY;
+	}
+
 	/* collect the session name if trailing args */
 	if (optind < argc)
 		server.session_name = argv[optind];
@@ -638,7 +645,7 @@ int main(int argc, char *argv[]) {
 	if (!action || !server.session_name)
 		usage();
 
-	if (tcgetattr(STDIN_FILENO, &orig_term) != -1) {
+	if (!passthrough && tcgetattr(STDIN_FILENO, &orig_term) != -1) {
 		server.term = orig_term;
 		has_term = true;
 	}

client.c

@@ -100,7 +100,8 @@ static int client_mainloop(void) {
 			if (client_recv_packet(&pkt)) {
 				switch (pkt.type) {
 				case MSG_CONTENT:
-					write_all(STDOUT_FILENO, pkt.u.msg, pkt.len);
+					if (!passthrough)
+						write_all(STDOUT_FILENO, pkt.u.msg, pkt.len);
 					break;
 				case MSG_RESIZE:
 					client.need_resize = true;