Configuration: made large_header_buffers a valid setting.

This is an extension to the previous commit, which made
large_header_buffer_size a valid configuration setting.

This commit makes a related value, large_header_buffers, a valid
configuration setting.

While large_header_buffer_size effectively limits the maximum size of
any single header (although unit will try to pack multiple headers into
a buffer if they wholly fit).

large_header_buffers limits how many of these 'large' buffers are
available. It makes sense to also allow this to be user set.

large_header_buffers is already set by the configuration system in
nxt_router.c it just isn't set as a valid config option in
nxt_conf_validation.c

With this change users can set this option in their config if required
by the following

    "settings": {
        "http": {
            "large_header_buffers": 8
        }
    },

It retains its default value of 4 if this is not set.

NOTE: This is being released as undocumented and subject to change as it
      exposes internal workings of unit.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

src/nxt_conf_validation.c

@@ -317,6 +317,9 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_http_members[] = {
     }, {
         .name       = nxt_string("large_header_buffer_size"),
         .type       = NXT_CONF_VLDT_INTEGER,
+    }, {
+        .name       = nxt_string("large_header_buffers"),
+        .type       = NXT_CONF_VLDT_INTEGER,
     }, {
         .name       = nxt_string("body_buffer_size"),
         .type       = NXT_CONF_VLDT_INTEGER,