Fixes and enchancements for enduser_setup module.

Fixes:

* Removed spurious \0 bytes being sent to the client in many cases.

* Properly terminated 404 header.

* Now including Content-length: header for index.html (including built-in).

  Clients otherwise had to wait for their own timeout before considering
  the page load complete, which resulting in an unpleasant sluggish
  experience.

* Switched form submission to GET, as iOS sometimes does not include the
  body in the same packet as the POST header, which led to the module
  not finding the form values.

* Added redirect after form submission to avoid blank-page-in-browser.

* Moved common "enduser_setup" prefix to a lua_pushfstring() to reduce
  string literal overhead.

* Replaced cached lua_State* with calls to lua_getstate(), as in other
  modules.

* Fixed broken appending of MAC to the SSID.

* Fixed up macro definitions to not break `if` clauses unintentionally.

Enhancements:

* Support for scanning for available WiFi networks, and picking from the list.

* Support for retrieving the current WiFi station status.

* Added "manual" mode, where the module does not (re)configure the AP setup,
  nor shut down automatically. Default is still automatic, as before.

* Updated docs for new `enduser_setup.manual()` function.

docs/en/modules/enduser-setup.md

@@ -6,6 +6,23 @@ This module provides a simple way of configuring ESP8266 chips without using a s
 After running [`enduser_setup.start()`](#enduser_setupstart) a portal like the above can be accessed through a wireless network called SetupGadget_XXXXXX. The portal is used to submit the credentials for the WiFi of the enduser.
 After an IP address has been successfully obtained this module will stop as if [`enduser_setup.stop()`](#enduser_setupstop) had been called.
 
+## enduser_setup.manual()
+
+Controls whether manual AP configuration is used.
+
+By default the `enduser_setup` module automatically configures an open access point when starting, and stops it when the device has been successfully joined to a WiFi network. If manual mode has been enabled, neither of this is done. The device must be manually configured for `wifi.SOFTAP` mode prior to calling `enduser_setup.start()`. Additionally, the portal is not stopped after the device has successfully joined to a WiFi network.
+
+Most importantly, *the `onConfigured()` callback is not supported in manual mode*. This limitation may disappear in the future.
+
+#### Syntax
+`enduser_setup.manual([on_off])`
+
+#### Parameters
+  - `on_off` a boolean value indicating whether to use manual mode; if not given, the function only returns the current setting.
+
+#### Returns
+The current setting, true if manual mode is enabled, false if it is not.
+
 ## enduser_setup.start()
 
 Starts the captive portal.
@@ -44,4 +61,4 @@ Stops the captive portal.
 none
 
 #### Returns
-`nil`
+`nil`