etags: add support for multiple tags

  Besides tag file in parent directories, also look for standard emacs
  locations stored in tags-file-name and tags-table-list
added comments on \\_<foo\\> default

   It is very annoying to edit \\_<foo\\_> on M-n, so use input.

   May be, first display using \\_<foo\\_> but insert "foo" in
   minibuffer. Is this possible?
* helm-tags.el (helm-etags-all-tag-files): Remove unused directory arg.
(helm-source-etags-select): Use mapc.