BUG/MEDIUM: mux-fcgi: Properly handle full buffer for FCGI_PARAM record

BUG/MEDIUM: mux-fcgi: Properly handle full buffer for FCGI_PARAM record

The function encoding and sending FCGI_PARAM records was reworked to
properly deal with full buffer. An error must be triggered only when the
parameters cannot be encoded while the mbuf is empty (or the free space is
greater than the max record size). Otherwise we must wait and retry later.

Before, an error was triggered on encoding error if any HTX block was
consumed, regarless the mbuf state. Now, blocks are removed on success
only. So we can wait for more space.

This patch should fix the issue #3346. It should be backported to all stable
versions.