[fpc-pascal] Windows find out if an application already is running

[dmitry boyarintsev]

it's better to use named mutexes (supported on all Windows versions)
to find out if there's another instance of the app running.

do the following on application start:
- create a named mutex (constant name)
- try to lock the mutex (0 time waiting), if failed then there's
another instance already running.

Thanks,
Dmitry