added support for multiple instances, tweaked screen execution

[billdotjr]

There are couple improvements:

1. Script now allows for running multiple instances of direwolf in parallel. (Can be useful with multiple RTL-SDRs.)
2. Direwolf is executed not as a string but as a script inside of the screen. (Passing of the pipe never really worked for me, the way it was implemented previously.) The script is the same script again, for keeping it simple.
3. rtl_fm params are now specified as variables for easier configuration for beginners.
4. The sleep time is now configurable. (Useful when troubleshooting.)

[dranch]

I think there are some interesting ideas in this patch (there are other pending patches have some similar fixes) but I also feel the dw-start.sh script should only be intended for basic needs. For people who want advanced uses such as multiple instances, run via SystemD, etc. should be managed by themselves. Just my $0.02.

[billdotjr]

Please be aware that the original file does not work on Raspbian. The pipe does not seem to get passed correctly.

Secondly, there is large crowd of people who want to set an Rx-only APRS iGate up and those are using the inferior pymultimonaprs because it is easier to set up. They find direwolf too complicated. I tried making it little more accessible for them.