bug#11702: 23.4; `grep-find-use-xargs' should be defined with `defcustom'

[Samuel Bronson]

I want to reassure Emacs.app that it's okay to use `xargs -0'.  The
`grep-find-use-xargs' variable seems perfect for this, but there's one
problem: it isn't defined using `defcustom', so it's actual *work* to
set it in an already-started Emacs.

(Actually, there's another problem -- Emacs tries to use `xargs -0 -e'
instead of `xargs -0', and BSD xargs doesn't support `-e' -- but that's
another bug.)