Switch ap_http_filter to use ap_get_brigade and apr_brigade_flatten

Switch ap_http_filter to use ap_get_brigade and apr_brigade_flatten
instead of ap_getline - this prevents some odd looping issues that
can cause problems.

Also, when we call get_mime_headers to read the trailers, we need
to reset our ctx->state to BODY_NONE - there should only be MIME-header
information (followed by a blank CRLF line) - and we don't know
how much data there will be - so it is by definition BODY_NONE.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94704 13f79535-47bb-0310-9956-ffa450edef68