Remove sorting headers from library code. The application must sort
them if necessary. nghttpx and nghttpd do the sorting of the headers
in stable way if names are equal.

nghttp2_session_client_new2 and nghttp2_session_server_new2 take
additional parameters which specifies session options.
nghttp2_set_option is somewhat crumsy because of type checking.
Now we use nghttp2_opt_set, which specifies individual options with
types. We changed the value of nghttp2_opt, so this change will
require re-compile.

Using --cacert to load certificate for client certificate authentication
is problematic since, --cacert is also used for client mode.
This commit adds --verify-client-cacert option which specify the CA
certficate file used only for client certificate validation.
This change also removes the default certficate load function for
client certificate validation.