Upgrade from 1.9.6 to 1.9.7.

<ChangeLog>

*) Feature: the "nohostname" parameter of logging to syslog.

*) Feature: the "proxy_cache_convert_head" directive.

*) Feature: the $realip_remote_addr in the ngx_http_realip_module.

*) Bugfix: the "expires" directive might not work when using variables.

*) Bugfix: a segmentation fault might occur in a worker process when

using HTTP/2; the bug had appeared in 1.9.6.

*) Bugfix: if nginx was built with the ngx_http_v2_module it was

possible to use the HTTP/2 protocol even if the "http2" parameter of
the "listen" directive was not specified.

*) Bugfix: in the ngx_http_v2_module.

</ChangeLog>