Security review: make privilege dropping-restoring optional.

This is/was necessary only on macOS, because you needed root in order
to read the process list. This was never necessary on Linux, and
it also raises security concerns, so now it needs to be enabled
explicitly at build time.

TraceScreen.c

@@ -96,7 +96,7 @@ bool TraceScreen_forkTracer(TraceScreen* this) {
    this->child = fork();
    if (this->child == -1) return false;
    if (this->child == 0) {
-      (void) seteuid(getuid());
+      CRT_dropPrivileges();
       dup2(this->fdpair[1], STDERR_FILENO);
       int ok = fcntl(this->fdpair[1], F_SETFL, O_NONBLOCK);
       if (ok != -1) {