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

 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