Send rtl_tcp header/magic number for compatibility

[theY4Kman]

This PR alters the protocol a bit: it sends an rtl_tcp-compatible header, including the RTL0 magic number, upon client connection.

This PR is a fix for #2

The header structure also contains a tuner type and count of gain values supported by the tuner. The tuner type is an enum, statically defined in rtl-sdr.h

enum rtlsdr_tuner {
	RTLSDR_TUNER_UNKNOWN = 0,
	RTLSDR_TUNER_E4000,
	RTLSDR_TUNER_FC0012,
	RTLSDR_TUNER_FC0013,
	RTLSDR_TUNER_FC2580,
	RTLSDR_TUNER_R820T,
	RTLSDR_TUNER_R828D
};

Since none of those quite fit, this PR simply uses 0 for unknown.

The tuner gain counts also doesn't quite fit — those tuners above appear to have statically defined sets of gain values:

int rtlsdr_get_tuner_gains(rtlsdr_dev_t *dev, int *gains)
{
	/* all gain values are expressed in tenths of a dB */
	const int e4k_gains[] = { -10, 15, 40, 65, 90, 115, 140, 165, 190, 215,
				  240, 290, 340, 420 };
	const int fc0012_gains[] = { -99, -40, 71, 179, 192 };
	const int fc0013_gains[] = { -99, -73, -65, -63, -60, -58, -54, 58, 61,
				       63, 65, 67, 68, 70, 71, 179, 181, 182,
				       184, 186, 188, 191, 197 };
	const int fc2580_gains[] = { 0 /* no gain values */ };
	const int r82xx_gains[] = { 0, 9, 14, 27, 37, 77, 87, 125, 144, 157,
				     166, 197, 207, 229, 254, 280, 297, 328,
				     338, 364, 372, 386, 402, 421, 434, 439,
				     445, 480, 496 };
	const int unknown_gains[] = { 0 /* no gain values */ };

And the Limes appear to accept 0-73dB, which would be 0-730 in rtl_tcp parlance... I guess 730 could be passed for gain counts, but I'm not sure what value it would add. This PR just uses 0 for that value, too.

[sally-yachts]

And the Limes appear to accept 0-73dB, which would be 0-730 in rtl_tcp parlance... I guess 730 could be passed for gain counts, but I'm not sure what value it would add. This PR just uses 0 for that value, too.

I tested with GQRX and it looks like this may prevent a gain slider from being presented to the end user. I can connect over the network but gain starts and remains at zero. When doing some testing with Nooelec NESDR SMArt v4 dongles with rtl_tcp I do get a gain slider and see the rtl_tcp output setting gain, although interestingly not using the r82xx_gains values above. Are there any command line switches to change the gain, or perhaps another client app (command line?) to test adjusting gain?