ignore SIGCHLD to prevent CGI zombies

with nothing waiting on the forked processes there is no reason not to
reap the CGI process on exit

intended to address issue from mailing list "gmnisrv uses too many
tasks" <163806E8-A16A-463E-8C62-43E903EF1E35@librem.one>

src/serve.c

@@ -5,6 +5,7 @@
 #include <fcntl.h>
 #include <libgen.h>
 #include <limits.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
@@ -141,6 +142,7 @@ serve_cgi(struct gmnisrv_client *client, const char *path,
 		return;
 	}
 
+	signal(SIGCHLD, SIG_IGN);
 	pid_t pid = fork();
 	if (pid == -1) {
 		server_error("fork: %s", strerror(errno));