Raise RLIMIT_NPROC only if maxnprocs higher than current limits

Otherwise maxnprocs may actually lower the limit. Especially when using the
default limit of 512, this quickly causes quark's fork() to fail when started
with a non-exclusive user.

To mitigate this, we respect system defaults and only raise the limit if it is
an actual raise.

main.c

@@ -285,7 +285,12 @@ main(int argc, char *argv[])
 	}
 
 	/* raise the process limit */
-	rlim.rlim_cur = rlim.rlim_max = maxnprocs;
+  if (getrlimit(RLIMIT_NPROC, &rlim) < 0) {
+    die("getrlimit RLIMIT_NPROC:");
+  }
+
+  rlim.rlim_cur = MAX(rlim.rlim_cur, maxnprocs);
+  rlim.rlim_max = MAX(rlim.rlim_max, maxnprocs);
   if (setrlimit(RLIMIT_NPROC, &rlim) < 0) {
     die("setrlimit RLIMIT_NPROC:");
   }