Limit threads to 1 when CPORT is set, closes rapid7#4170

This issue also applies to TCP scanner modules.

Author: HD Moore

lib/msf/core/auxiliary/scanner.rb

@@ -59,9 +59,15 @@ def run
   @tl = []
 
   #
-  # Sanity check threading on different platforms
+  # Sanity check threading given different conditions
   #
 
+  if datastore['CPORT'].to_i != 0 && threads_max > 1
+    print_error("Warning: A maximum of one thread is possible when a source port is set (CPORT)")
+    print_error("Thread count has been adjusted to 1")
+    threads_max = 1
+  end
+
   if(Rex::Compat.is_windows)
     if(threads_max > 16)
       print_error("Warning: The Windows platform cannot reliably support more than 16 threads")