Modules: added jsVarNames() method.

[xeioex]

The method returns names of variables declared with js_var. An optional string argument filters returned names by prefix.

This lets JavaScript code discover a configuration-defined variable schema without duplicating every variable name in the script. For example:

js_var $extract_name;
js_var $extract_arguments_workspace;

function handler(r) {
    r.jsVarNames("extract_").forEach(function(name) {
        r.variables[name] = extract(name.slice(8));
    });
}

The same API is available as s.jsVarNames() in stream. Variables created by js_set or by the core modules are not returned.

[VadimZhestikov]

Minor suggestions:

---

Stream module implementations

Fully symmetric to the HTTP implementations — ngx_stream_session_t, ngx_stream_core_main_conf_t,
ngx_stream_js_variable_var used in place of their HTTP counterparts. Logic is identical.

Minor inconsistency between the two QJS variants — n = 0 is initialised at the declaration site in the
HTTP version but near its first use in the stream version:

  // HTTP QJS:
      n = 0;
      cmcf = ngx_http_get_module_main_conf(r, ngx_http_core_module);
      v = cmcf->variables.elts;

  // Stream QJS:
      cmcf = ngx_stream_get_module_main_conf(s, ngx_stream_core_module);
      v = cmcf->variables.elts;
      n = 0;

Not a bug, but worth harmonizing.

---

Tests:

1. jsVarNames(null) — both implementations would throw TypeError (null is not undefined, not a string),
   but this isn't tested.
2. jsVarNames(undefined) explicitly passed — both implementations treat it the same as no argument
   (returns all), but not tested explicitly. Low priority.

[VadimZhestikov]

Looks good

[VadimZhestikov]

Looks good