MEDIUM: mux-h1: Simplify the connection mode management by sanitizing headers

MEDIUM: mux-h1: Simplify the connection mode management by sanitizing headers

Connection headers are now sanitized during the parsing and the formatting. This
means "close" and "keep-alive" values are always removed but right flags are
set. This way, client side and server side are independent of each other. On the
input side, after the parsing, neither "close" nor "keep-alive" values
remain. So on the output side, if we found one of these values in a connection
headers, it means it was explicitly added by HAProxy. So it overwrites the other
rules, if applicable. Always sanitizing the output is also a way to simplifiy
conditions to update the connection header. Concretly, only additions of "close"
or "keep-alive" values remain, depending the case.

No need to backport this patch.