http: add ap_fgetline() and AP_GETLINE_NONBLOCK flag.

http: add ap_fgetline() and AP_GETLINE_NONBLOCK flag.

It allows to read a line directly from an input filter, in blocking mode
or not. Since no request_rec is needed, a pool may be given.

Existing ap_[r]getline() function are now based off ap_fgetline() by calling:
    ap_fgetline(s, n, read, r->proto_input_filters, flags, bb, r->pool);

Will follow up with a new ap_get_mime_headers_*() flavor which can be used by
any filter that needs non-blocking and not necessarily has a request_rec (e.g.
ap_http_filter() to read proxied response trailers).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1829659 13f79535-47bb-0310-9956-ffa450edef68