Rewrite ap_rgetline to remove the need to have an "internal" brigade stored

Rewrite ap_rgetline to remove the need to have an "internal" brigade stored
in the core_module structure by using the AP_MODE_SPECULATIVE filter mode
to determine if MIME-continuation should occur.

Notes:
- ap_rgetline has a new prototype.
- ap_rgetline returns APR_ENOSPC when we are out of buffer space.

All direct callers of ap_rgetline are now adjusted to handle this new API.
ap_getline will mimic the old API for now.

Reviewed by:	Ryan Morgan

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93011 13f79535-47bb-0310-9956-ffa450edef68