Provides a common function, url.get_default_port(), for obtaining

the default port number for a given scheme. Fixes #781

nselib/data/http-default-accounts-fingerprints.lua

@@ -175,10 +175,8 @@ local function url_build_defaults (host, port, parsed)
   local parts = tcopy(parsed or {})
   parts.host = parts.host or stdnse.get_hostname(host, port)
   parts.scheme = parts.scheme or shortport.ssl(host, port) and "https" or "http"
-  local pn = parts.port or port.number
-  if not (parts.scheme == "http" and pn == 80
-       or parts.scheme == "https" and pn == 443) then
-    parts.port = pn
+  if not parts.port and port.number ~= url.get_default_port(parts.scheme) then
+    parts.port = port.number
   end
   return parts
 end