MEDIUM: config: ensure that tune.bufsize is at least 16384 when using HTTP/2

MEDIUM: config: ensure that tune.bufsize is at least 16384 when using HTTP/2

HTTP/2 mandates the support of 16384 bytes frames by default, so we need
a large enough buffer to process them. Till now if tune.bufsize was too
small, H2 connections were simply rejected during their establishment,
making it quite hard to troubleshoot the issue.

Now we detect when HTTP/2 is enabled on an HTTP frontend and emit an
error if tune.bufsize is not large enough, with the appropriate
recommendation.