cgi-io: use dynamic memory for post decoding, support proc files

Allocate dynamic buffer memory for decoding post data and allow post
requsts up to 128KB compared to the previos 1KB limit.

Also support downloading /proc and /sys files by falling back to
chunked transfer encoding when the file size cannot be determined.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 39087eba)