use them if you got 'em

bugfix: callback() now called for some kinds of errors
bugfix: 'select' module not imported when using CLI
bugfix: CLI 'python -m xmodem send', recv still not implemented!

remove code and CLI options that aren't implemented, we should probably
implement the 'xmodem send' sub-command soon ..

change: remove unused runx() function
change: remove the serial arguments and module import they were never implemented!
change: set 'send error:' msg loglevel info -> error
change: set 'error_count reached' msg loglevel info -> error
change: set 'recv error: putc failed' msg loglevel debug -> warning
change: remove unused 'char' variable

- make 'lrzsz' package optional, using pytest.mark.skipif()
- add .editorconfig
- add pytest.ini

remove `test/functional/test.py`, somewhat useful ideas that have since been
implemented in automatic pytests. This is out-of-date py2-only code, about
bytes/ord/chr, StringIO(), and some unused ideas like simulating modem jitter.

add a few specific tests, for more systemic coverage, it is a good idea to use
these callbacks in automatic tests, too.