(query-replace-read-args): Only warn about use of \n

(query-replace-read-args): Only warn about use of \n
and \t when we are doing a regexp replacement and the actual
escaped character is n or t.
(query-replace-regexp): Add \, and \# interpretation to
interactive call and document it.
(query-replace-regexp-eval, replace-match-string-symbols): add \#
as shortkey for replace-count.
(replace-quote): New function for doubling backslashes.