BUG/MINOR: config: better detect the presence of the h2 pattern in npn/alpn

BUG/MINOR: config: better detect the presence of the h2 pattern in npn/alpn

In 1.8, commit 45a66cc ("MEDIUM: config: ensure that tune.bufsize is at
least 16384 when using HTTP/2") tried to avoid an annoying issue making
H2 fail when haproxy is built with default buffer sizes smaller than 16kB,
which used to be the case for a very long time. Sadly, the test only sees
when NPN/ALPN exactly match "h2" and not when it's combined like
"h2,http/1.1" nor "http/1.1,h2". We can safely use strstr() there because
the string is prefixed by the token's length (0x02) which is unambiguous
as it cannot be part of any other token.

This fix should be backported to 1.8 as a safety guard against bad
configurations.