Function: find-grep-dired

Find files in DIR containing a regexp REGEXP and start Dired on output.
The command run (after changing into DIR) is

find . \( -type f -exec `grep-program' `find-grep-options' \
-e REGEXP {} \; \) -ls

where the car of the variable `find-ls-option' specifies what to
use in place of "-ls" as the final argument. (fn DIR REGEXP)