Added max_headers directive.

Added max_headers directive.

The directive limits the number of request headers accepted from clients.
While the total amount of headers is believed to be sufficiently limited
by the existing buffer size limits (client_header_buffer_size and
large_client_header_buffers), the additional limit on the number of headers
might be beneficial to better protect backend servers.

Requested by Maksim Yevmenkin.

Signed-off-by: Elijah Zupancic <e.zupancic@f5.com>
Origin: <https://freenginx.org/hg/nginx/rev/199dc0d6b05be814b5c811876c20af58cd361fea>