Incorporate updates from xterm #336 and vttest 20180911

Created by: ThomasDickey

* README.txt: document --xterm-checksum

* escargs.py: add --xterm-checksum option

* esc.py: modify empty() to use option --xterm-checksum

* escutil.py:
if xterm is (expected to) return DEC-compatible checksums, transform the
result to make it compatible with esctest's assumptions.

* escio.py, tests/cht.py, esccmd.py, tests/decfi.py: pylint fixes