ngx-active-reqs: fix the "semantic error: symbol without referent: id…

[yancl]

when run ngx-active-reqs by stap 4.0 without the "-k" option, it reports the following errors:

semantic error: symbol without referent: identifier 'nreqs' at <input>:133:13
    source:         if (nreqs || !0) {
                            ^

so we can assign the $keep_waiting const to a var to fix it

[yancl]

also fix https://github.com/openresty/openresty-systemtap-toolkit/issues/47 and https://github.com/openresty/openresty-systemtap-toolkit/issues/55