Refactor serial port usage

[v3rm0n]

• Move all references to libserialport to serial.c
• Move the serial port instance to serial.c and don't expose it
• Define USE_LIBSERIALPORT flag to turn libserialport dependent code on/off

Closes #91

[v3rm0n]

Actually I would like to change it so that libserialport is always the default and I'll add libusb implementation which can be turned on by specifying USE_LIBUSB

[v3rm0n]

I'm marking it as a draft for now beecause I want to improve it a bit. I don't like the way that the filehandle works currently and it's pretty hard to do other libusb related stuff (like audio). Will work on it a bit more.

[v3rm0n]

I think it looks better now.

[laamaa]

Thanks for the PR! This is actually more how I originally wanted to make the program work, but didn't have the skills.. :)

Tested by editing makefile manually, adding libusb-1.0 to pkg-config parameters and compiling with CFLAGS=-DUSE_LIBUSB, seemed to work fine with that and by compiling "normally". With libusb there were occasional glitches in the graphics, but nothing too big.

A few thoughts:

• Do you think it's better to have the both serial access systems in one file (serial.c), or would it be more clear if they were in separated into two files?
• Would it be possible to add a simpler way to compile the program with libusb, like by using a different make parameter, eg. "make libusb"?
• Using SIGQUIT isn't supported under Windows, it should probably be checked if it's defined first

[v3rm0n]

Probably some parameters need a bit of tuning to get it to work optimally. I just used whatever started to work (like #define serial_read_size 512)

[laamaa]

Thank you!! :)