Static: variables in the "share" option.

This commit supports variable in the "share" option, the finding path to
file serve is the value from "share". An example:
{
    "share": "/www/data/static$uri"
}

src/nxt_conf_validation.c

@@ -634,6 +634,7 @@ static nxt_conf_vldt_object_t  nxt_conf_vldt_share_action_members[] = {
     {
         .name       = nxt_string("share"),
         .type       = NXT_CONF_VLDT_STRING,
+        .flags      = NXT_CONF_VLDT_VAR,
     }, {
         .name       = nxt_string("types"),
         .type       = NXT_CONF_VLDT_STRING | NXT_CONF_VLDT_ARRAY,