Added systemd service file, added install stanza to Makefile.

[K2IE]

Install stanza will perform system install when PREFIX is set. The dump1090.service file will make use of an /etc/default/dump1090 file which could contain a line similar to the following:

Options to pass to dump1090

DUMP1090_OPTS="--quiet --net --lat 40.12345 --lon -70.54321 --ppm 19 --gain -10"

With the dump1090.service enabled the process will start upon reboot and can be managed via systemd start and stop commands.