add some security checks when running SUID root

on Darwin, htop needs to run with root privileges to display information
about other users processes. This commit makes running htop SUID root a
bit more safe.

TraceScreen.c

@@ -86,6 +86,7 @@ void TraceScreen_run(TraceScreen* this) {
    int child = fork();
    if (child == -1) return;
    if (child == 0) {
+      seteuid(getuid());
       dup2(fdpair[1], STDERR_FILENO);
       int ok = fcntl(fdpair[1], F_SETFL, O_NONBLOCK);
       if (ok != -1) {