Dynamic modules.

The auto/module script is extended to understand ngx_module_link=DYNAMIC.
When set, it links the module as a shared object rather than statically
into nginx binary.  The module can later be loaded using the "load_module"
directive.

New auto/module parameter ngx_module_order allows to define module loading
order in complex cases.  By default the order is set based on ngx_module_type.

3rd party modules can be compiled dynamically using the --add-dynamic-module
configure option, which will preset ngx_module_link to "DYNAMIC" before
calling the module config script.

Win32 support is rudimentary, and only works when using MinGW gcc (which
is able to handle exports/imports automatically).

In collaboration with Ruslan Ermilov.

auto/summary

@@ -60,6 +60,7 @@ echo
 cat << END
   nginx path prefix: "$NGX_PREFIX"
   nginx binary file: "$NGX_SBIN_PATH"
+  nginx modules path: "$NGX_MODULES_PATH"
   nginx configuration prefix: "$NGX_CONF_PREFIX"
   nginx configuration file: "$NGX_CONF_PATH"
   nginx pid file: "$NGX_PID_PATH"