gitweb: Separate features with no project specific override

gitweb: Separate features with no project specific override

Put features for which project specific override is not supported into
separate %global_feature hash, updating gitweb_check_feature()
subroutine to use it.  For backward compatibility with existing gitweb
configuration setups $feature{<feature>}{'default'} has preference
over $global_feature{<feature>}.

While at it remove stale comment for 'pathinfo' feature; requiring to
use workaround specified was fixed in b65910f (gitweb: remove
PATH_INFO from $my_url and $my_uri).

FIX IT: some spurious "HASH.." is printed to STDERR. (???)

Signed-off-by: Jakub Narebski <jnareb@gmail.com>