Add reset capability to emulator modules

[giomba]

These patches add the reset capability to the various emulator modules.
Then, the reset command is introduced in the command line to perform a cold boot.

I did not check thoroughly the documentation of every single module (eg. the SIO/2 or the CRTC) but I just replicated the behaviour of a standard boot in the emulator (eg. reinitialize static variables with their normal startup value).

@giuliof please have a look at the modified fdc.c module to check if everything looks okay.
Last but not least, I fiddled a bit with reset and multiple floppy disks, and it does not always behave the same way as the actual machine, but I suspect some variable initialization problem which (of course) was not diagnosed before because the reset did not exists. So I'm opening a follow up issue #29

[giuliof]

Code is ok, but I have an (I hope) better proposal to emulate what happens at reset.

The two proposals are in branches giuliof/reset-1 and giuliof/reset-2

[giomba]

Code is ok, but I have an (I hope) better proposal to emulate what happens at reset.

Thank you for your input. I was focused on the general architecture for the reset, that I did not actually fix what happens specifically on the bus. I integrated your commit with this MR, and I'll merge it.