built-in key lists for keys command should be more systematic

[dmolesUC]

The current set of key lists is fairly random and ad-hoc:

1. Default: default source (as naughty-strings, filtering out "", ".", and leading "..", all of which are known to fail in Amazon S3) (500 keys)
2. disallow-backslash: as default source, disallowing backlash (320 keys)
3. disallow-double-backslash: as default source, disallowing double backlash (498 keys)
4. misc: miscellenous potential problems, incl. path elements & unicode blocks (612 keys)
5. naughty-strings: Big List of Naughty Strings (504 keys)

The naughty-strings list is fairly reasonable, although not all that directly applicable to filenames; it tends to produce a lot of redundant failures since many of its Javascript escapes and whatnot are very similar. A better, shorter list could probably be made by boiling it down to more general cases.

The misc list is pretty ad-hoc and doesn't cover every Unicode code block, or even most of them.

We should come up with some systematic cases and some systematic lists based on those cases.

[dmolesUC]

Also, they should include some strings that are bytewise valid in other charsets (esp. historical charsets) but not in UTF8.

[dmolesUC]

The cos suite --unicode test now covers all Unicode characters, systematically. It does attempt to test invalid UTF8 sequences, but it's not clear how effective that test is (e.g. it may be sensitively dependent on how the AWS client library converts Go strings to JSON).