Since http_req_size was already established for this purpose, and is
now enforced for h2 traffic, it should naturally become the basis for
the MAX_HEADER_LIST_SIZE setting in the initial SETTINGS frame sent
to clients.

The h2_max_header_list_size parameter will grow a new purpose.

Conflicts:
	bin/varnishtest/tests/t02000.vtc
	bin/varnishtest/tests/t02005.vtc
	include/tbl/params.h