librtlsdr: new package

librtlsdr is a library and software for using a Realtek RTL2832U based
DVB-T dongle as a cheap SDR.

[Thomas: change license to GPLv2+, reword help text for the detach
driver option.]

Signed-off-by: Jason Pruitt <jrspruitt@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>