MEDIUM: tune.http.maxhdr makes it possible to configure the maximum number of HTTP...

MEDIUM: tune.http.maxhdr makes it possible to configure the maximum number of HTTP headers

For a long time, the max number of headers was taken as a part of the buffer
size. Since the header size can be configured at runtime, it does not make
much sense anymore.

Nothing was making it necessary to have a static value, so let's turn this into
a tunable with a default value of 101 which equals what was previously used.