Add stop for exceeding maximum frequency separation

[E3V3A]

Since the RTL-SDR cannot fetch more than 2 MHz bandwidth, the frequencies provided in the list may not be separated by more than this. However, the binary happily accepts anything, and continues execution with unknown consequences.

I propose to implement a stop for this.

[E3V3A]

Rewrite this:

if ((Fd[nbch - 1] - Fd[0]) > RTLINRATE - 4 * INTRATE) {
		fprintf(stderr, "Frequencies too far apart\n");
		return -1;
}

To:

if ((Fd[nbch - 1] - Fd[0]) > RTLINRATE - 4 * INTRATE) {
		fprintf(stderr, "ERROR: Frequencies are too far apart: df=%.2f MHz\n", (float)(Fd[nbch - 1] - Fd[0]));
		exit(1);
}

PS. I compiled this in my head, so I haven't tested it...

This also fixes issue (2) in #20.

[TLeconte]

Ok thanks fix it by just returning 0 in ChooseFc (returning -1 is not a good idea for an unsigned int return function)