Headers filter: inheritance control for add_header and add_trailer.

Headers filter: inheritance control for add_header and add_trailer.

The new directives add_header_inherit and add_trailer_inherit allow
to alter inheritance rules for the values specified in the add_header
and add_trailer directives in a convenient way.

The "merge" parameter enables appending the values from the previous level
to the current level values.

The "off" parameter cancels inheritance of the values from the previous
configuration level, similar to add_header "" (2194e75bb).

The "on" parameter (default) enables the standard inheritance behaviour,
which is to inherit values from the previous level only if there are no
directives on the current level.

The inheritance rules themselves are inherited in a standard way.  Thus,
for example, "add_header_inherit merge;" specified at the top level will
be inherited in all nested levels recursively unless redefined below.