Explain why lseek() is required in the Linux code.
Revert "Move operating system detection to input.h so that other programs can"

This reverts commit 9872cd25d26b34757c7c719a87541ca7e5a07e0d.

Conflicts:
	input.h
Implement working version (from readpin.c) in get_bit(), use relative
values for margin and logical '1' detection.
Cosmetics
Change meaning of margin from absolute samples to relative error margin
for '0' and '1' detection (default now 2%)
Use get_bit() and set_mode() in readpin.c, now a very short program.
Update math example for new meaning of hw.margin.
Reduce #ifdef clutter.