MEDIUM: h1: Add an option to sanitize connection headers during parsing

MEDIUM: h1: Add an option to sanitize connection headers during parsing

The flag H1_MF_CLEAN_CONN_HDR has been added to let the H1 parser sanitize
connection headers. It means it will remove all "close" and "keep-alive" values
during the parsing. One noticeable effect is that connection headers may be
unfolded. In practice, this is not a problem because it is not frequent to have
multiple values for the connection headers.

If this flag is set, during the parsing The function
h1_parse_next_connection_header() is called in a loop instead of
h1_parse_conection_header().

No need to backport this patch