--eliot-destination reservation of "\" for escape is incompatible with typical Windows usage

[exarkun]

[ERROR]
Traceback (most recent call last):
  File "c:\projects\tahoe-lafs\.tox\py\lib\site-packages\unittest2\case.py", line 67, in testPartExecutor
    yield
  File "c:\projects\tahoe-lafs\.tox\py\lib\site-packages\unittest2\case.py", line 625, in run
    testMethod()
  File "c:\projects\tahoe-lafs\src\allmydata\test\test_eliotutil.py", line 471, in test_regular_file
    _parse_destination_description("file:{}".format(path))(reactor),
  File "c:\projects\tahoe-lafs\src\allmydata\util\eliotutil.py", line 410, in parse
    return parser(kind, args)
  File "c:\projects\tahoe-lafs\src\allmydata\util\eliotutil.py", line 426, in _parse_file
    u"Unsupported escape character (\\) in destination text ({!r}).".format(arg_text),
exceptions.ValueError: Unsupported escape character (\) in destination text (u'\\\\?\\C:\\projects\\tahoe-lafs\\_trial_temp\\allmydata.test.test_download\\DownloadV2\\test_download_no_overrun\\pjsjzk\\temp\\clients\\ol2lnqfj\\tmp\\tmps1zefj\\regular_file').
allmydata.test.test_eliotutil.ParseDestinationDescriptionTests.test_regular_file

[GitHub <noreply@…>]

In 64ef320/trunk:

Merge pull request #557 from tahoe-lafs/2980.eliot-destination-escaping

Fix escaping in Eliot destinations

Fixes: ticket:2980