Slightly improved DEBUG logging
When viewing debug logs, add a little more code to show the file & line number of the calling function, to aid debugging.
Note that this implementation (taking a look at the calling stack) will make running ISPConfig3 to run slightly slower, but if one is debugging ISPConfig3, I guess one will be willing to accept a small loss of performance...
(Posted here to correctly add a Merge Request, assuming that there is interest in this snippet of code...)