Add server tokens back to 2.0. Also bring forward the change to allow

Add server tokens back to 2.0.  Also bring forward the change to allow
the PRODUCT_ONLY value for ServerTokens.  This is relatively clean,
all of the code lives in http_core, and when a module wants to add a token,
they just call ap_add_version_component from the post_config hook.  Actually
ap_add_version_component can be done anytime after the config has been
parsed, it just makes the most sense to do it in post_config IMHO.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85426 13f79535-47bb-0310-9956-ffa450edef68