BUG/MEDIUM: mux-fcgi: Try to fully fill demux buffer on receive if not empty

BUG/MEDIUM: mux-fcgi: Try to fully fill demux buffer on receive if not empty

Don't reserve space for the HTX overhead on receive if the demux buffer is
not empty. Otherwise, the demux buffer may be erroneously reported as full
and this may block records processing. Because of this bug, a ping-pong loop
till timeout between data reception and demux process can be observed.

This bug was introduced by the commit 5f927f603 ("BUG/MEDIUM: mux-fcgi:
Properly handle read0 on partial records"). To fix the issue, if the demux
buffer is not empty when we try to receive more data, all free space in the
buffer can now be used. However, if the demux buffer is empty, we still try
to keep it aligned with the HTX.

This patch must be backported to 3.1.