Discourage too-long usernames.

Usernames longer than 20 chars look like they aren't mnemonic,
so discourage these longer names.

Also, discourage names that look like possible email addresses
(ending with a common domain suffix).
More informative message when rejecting "myemailatschooldotedu".
Merge pull request #80 from PencilCode/shortnames

Discourage too-long usernames.