comms/dcf77pi: update to 2.0.2

• Prevent a possible string overflow when reading the configuration file.
• Plug a few memory leaks.
• Refactor a lot of code.

• Replace -l parameter (specify logfile) of dcf77pi by an interactive 'L' key which can also be used to change or disable the log file.
• New entry in config.txt, "outlogfile", to specify the the default log file (default empty).

• Fix a bug where the hour would be wrongly considered off by one at the moment of a DST change.

• Various bug fixes in the GUI code (only affected displaying of the data).

• Handle log files with \n\r line endings used by RISC OS (which runs on the Raspberry Pi).
• Slightly improve output when reading from a log file.

• Update README.md