www/{nginx,nginx-devel,freenginx}: 3rd-party modules management

Rename www/lua-stream-nginx-module to www/nginx-module-lua-stream
to follow the nginx-module-* naming scheme for dedicated 3rd-party
module ports and adjust the consumers. Relax the BUILD_DEPENDS to
an unversioned dependency while here, matching the LUA option.

While here, rename the internal NJS_QJS_CFLAGS variable to
_NJS_QJS_CFLAGS so it is no longer treated as an option helper.

Sponsored by: Netzkommune GmbH