Dynamic modules: auto/module script.

Dynamic modules: auto/module script.

This script simplifies configuration of additional modules,
including 3rd party ones.  The script is extensible, and
will be used to introduce dynamic linking of modules in upcoming
changes.

3rd party module config scripts are called with ngx_module_link
preset to "ADDON" - this allows config scripts to call auto/module
without ngx_module_link explicitly defined, as well as testing if
new interface is in place if compatibility with older nginx versions
is desired.

In collaboration with Ruslan Ermilov.