BUG/MEDIUM: h3: Properly report a C-L header was found to the HTX start-line

BUG/MEDIUM: h3: Properly report a C-L header was found to the HTX start-line

When H3 HEADERS frames are converted to HTX, if a Content-Length header was
found, the HTX start-line must be notified by setting HTX_SL_F_CLEN flag.
Some components may rely on this flag to know there is a content-length
without looping on headers to get the info.

Among other this, it is mandatory for the FCGI multiplexer because it must
announce the message body length.

This patch must be backported as far as 2.6.