Rewrite ap_get_client_block to rely on assumptions that have been

Rewrite ap_get_client_block to rely on assumptions that have been
solidified after this code was originally written.  Namely:

- AP_MODE_READBYTES will only return a brigade representing AT MOST bytes
  of data.  It can NOT return MORE than requested.
- APR_BLOCK_READ is respected - it is considered a design error of a filter
  if it returns without reading something.
- apr_brigade_flatten is available to do the heavy lifting of the copying
  into a flat buffer (as hinted at by the removed comment).

Tested with httpd-test.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95721 13f79535-47bb-0310-9956-ffa450edef68