adding function to read if apreq has been started

Added a function that can be used to test if apreq has been started. To test, make a script like this:

    print(apreqStarted())
    print(POST)
    print(apreqStarted())

And the call it with curl:

    jeroen@Ubuntu:~$ curl http://localhost/R/aptest -d 'foo=bar'
    [1] FALSE
    $foo
    [1] "bar"
    
    [1] TRUE

Alternatively, if we use a content type that libapreq doesn't support:

    jeroen@Ubuntu:~$ curl http://localhost/R/aptest -d 'foo=bar' -H "Content-Type: application/json"
    [1] FALSE
    NULL
    [1] FALSE

I.e. the script correctly tells us if apreq has been invoked yet.