(grep-template): Add "<X>" to docstring.

(grep-template): Add "<X>" to docstring.
(grep-files-aliases): Add "all".  Move "el" and "ch" to the top of
the list.  Move "asm" to the bottom.
(grep-find-ignored-directories): Add `choice' with nil value
to empty the list easily.
(grep-find-ignored-files): New option.
(grep-files-history): Set to nil by default instead of '("ch" "el").
(grep-compute-defaults): Add "<X>" to `grep-template'.
(grep-read-files): Bind new local variables `default-alias' and
`default-extension'.  Use a list of default values for the file prompt.
(lgrep): Add `--exclude=' command line options composed from
`grep-find-ignored-files'.
(rgrep): Add `-name' command line options composed from
`grep-find-ignored-files'.  (Bug#4301)