Fix a bug in the input handling. ap_http_filter() was modifying *readbytes

Fix a bug in the input handling. ap_http_filter() was modifying *readbytes
which corresponded to r->remaining (in ap_get_client_block). However,
ap_get_client_block was *also* adjusting r->remaining. Net result was that
PUT (and probably POST) was broken. (at least on large inputs)

To fix it, I simply removed the indirection on "readbytes" for input
filters. There is no reason for them to return data (the brigade length is
the return length). This also simplifies a number of calls where people
needed to do &zero just to pass zero.

I also added a number of comments about operations and where things could be
improved, or are (semi) broken.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89008 13f79535-47bb-0310-9956-ffa450edef68