http_header_access & header_replacement bugfix

Whilst working on dynamic registration of parsing types & names, I
noticed that the above are slightly borken.

Specifically neither produce output that can be put into a squid.conf
file...

Attached is a fix. It may not be the cleanest, and I haven't build
tested it (I have in the generic_modules branch, but the function
definitions are a little different (void * at the end) so I've
backported this to squid-head (which is where the patch is against).

Rob