#188 Isolate platform independent code in monitor.c, consolidate API for starting monitor.

[Gleb Smirnoff]

Hi! Can you please take a look at my pull request? It continues to the work for multiplatform support that we started in 2018.

https://sourceforge.net/p/minidlna/git/merge-requests/23/

P.S. Looks like git pull requests are not listed in the ticket queue, so decided to create a ticket.