Convert psql's tab completion for backslash commands to the new style.

This requires adding some more infrastructure to handle both case-sensitive
and case-insensitive matching, as well as the ability to match a prefix of
a previous word.  So it ends up being about a wash line-count-wise, but
it's just as big a readability win here as in the SQL tab completion rules.

Michael Paquier, some adjustments by me