atcommander: API/CLI consistency improvements

This patch contains several changes to make both the API and CLI
more consistent as well as minor code cleanup, including:
 - Added param 15 (MAX_WINDOW)
 - Moved CLI-only module imports into "if __name__ ..." section
 - Allow user-specified serial read timeout
 - Always enter command mode in "local" mode
 - Added "__" to class data members
 - Class methods now have consistent return values:
   - Methods that "do" things return True or False
   - Methods that return data either return that data or None
   - Exceptions should be handled within class methods
 - Int/float queries support negative numbers
 - Exact query supports re patterns and strips trailing newline
 - 1-second waits for entering command mode are properly set
 - Slightly more aggressive ATO and ATZ usage
 - get_radio_version() handles trailing non-numeric characters
   (hattip github user mw46d for fixing this a different way)
 - Consistent CLI exit codes that report all interesting errors
 - Shortened CLI arguments for list and set operations
 - Several CLI operations are implicitly retried
 - Radio parameters are listed using the same names used to set them
 - Indenting and trailing whitespace cleaned up

Author: Mike Clement