Commit 2 on 6 to fix reentrance (incomplete Zlib header or validation bytes) in mod_d...

Commit 2 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 input filter :
- loop until all the header is received,
- handle non blocking reads returning empty brigade,
- fix a double ap_get_brigade() when an EOS brigade is encountered while reading the header,
- in that case and no data was received so far, don't return an error but SUCCESS with the EOS, otherwise fail,
- don't remove the Content-Length and Content-MD5 headers until some data is read.

Still does not handle Zlib flags for now, next commits.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1572663 13f79535-47bb-0310-9956-ffa450edef68