Whitespace police.
Add a new hardware parameter: realfreq

This parameter will be used in the new bit detection algorithm and will
replace the minlen and maxlen parameters.
Replace the bit detection algorithm by an idea from Udo Klein, with permission.

For a raw pulse log which is 5.7 days long the number of wrongly detected
minutes dropped from ~2000 (depending on the value of minlen) to ~20.

The realfreq parameter should be set to the average value of thigh seen
so that the new bit starts at approximately 50% of the rising pulse.

Link readpin against libm
Update the frequency parameter for the new algorithm.
Hide the nanosleep time tuning behind #ifdef TUNETIME

In practice it does not have a noticable effect (maybe due to nanosleep()
sleeping for *at least* the specified time) so it mostly complicates code.

While here show negative values for twait, they sometimes happen.
Note why we cannot write 1e9 here...
- Detect the bitvalue (0, 1, or 2 (error) ) and use a maximum value for
  the 0 and 1 values instead of one margin value. Update config.[ch]
  and input.[ch] accordingly.
- Remove unnecessary thigh and better specify some other variables.
- stv is now unsigned, so use 2 instead of -1 as initial value.
Update config.txt for the new bit value parameters, and remove now unused minlen and maxlen.
Add detection for timeouts and remove a finished TODO
Use hw.maxzero and hw.maxone instead of hw.margin in input.c and drop EOM tail case.
Remove an unneeded variable.
Replace usleep() by nanosleep() to guarantee the minimum time period in case of an interruption.
Use the idea from Udo Klein to decode the bits instead of our own home-grown algorithm.

Drop remaining usage of hw.margin, hw.minlen, and hw.maxlen

Link dcf77pi against libm
By definition, a timeout is now 2500 ms long, so drop the approximation.
Format verbose output into columns for readability.
Update comments in get_bit() to the new algorithm / hardware parameters.
Update README.md for the new bit decoding algorithm / hardware parameters.
Add back support for detecting transmitter (all high) and receiver (all low) errors.