Handle regexp and wildcard search terms

Handle regexp and wildcard search terms

* lisp/gnus/gnus-search.el (gnus-search-query-return-string): Fix up
  this function to be a little more general. Quoted strings are now
  returned with quotes.
  (gnus-search-run-search): Pick up and (partially) use the FUZZY IMAP
  capability.
  (gnus-search-transform-expression): In IMAP, check for wildcards and
  turn them into FUZZY as appropriate. Drop regexps.
  (gnus-search-indexed-massage-output):
  (gnus-search-transform-expression): In Notmuch, only drop leading
  asterisks.
* test/lisp/gnus/search-tests.el (gnus-s-delimited-string): Add test
  for `gnus-search-query-return-string'.