Commit 1 on 6 to fix reentrance (incomplete Zlib header or validation bytes) in mod_d...

Commit 1 on 6 to fix reentrance (incomplete Zlib header or validation bytes) in mod_deflate's output and input filters.
PR 46146 (patches from duplicated PR 55666)

Handle Zlib header buffering in the inflate output filter :
- add the new deflate_ctx_t fields needed to re-enter the Zlib header parsing,
- introduce the new consume_zlib_flags() function to parse/consume the ZLib flags (will be used by the other filters too),
- use it to handle incomplete header in the output filter (deflate).

This alone fixes PR 55666, but the issue remains for PR 46146 (inflate/deflate input filters), hence the following patches.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1572655 13f79535-47bb-0310-9956-ffa450edef68