Fix building with headers more module

I stumbled over the issue of not being able to build nginx-full --with-header-more-module. I found out the underlying problem was that a mix of the singular version (header more module) and plural version (header**s** more module) was in use.

The official name for the module is the plural version. Thus I propose to change names to the plural version.

This PR changes built flag --with-header-more to --with-headers-more, updates all remaining references and fixes build when building with headers more module.