bogus changes to executable.el, grep.el

[Dave Love]

M-x grep-find fails with `void function
executable-command-find-unix-p'.  I assume this is meant to be
`executable-command-find-posix-p', but that's bogus anyhow.

I don't know exactly what it's supposed to do, or why, but using a
non-POSIX option can't test positively for a plausible interpretation
of `handles arguments Posix-style'.  It also uses CL function
`find-if'.  If it needs to check the operation of `find' for some
reason on a file it knows exists, I'd expect it just to use
`shell-command-to-string' with something like etc/NEWS in a defvar (as
normal).

`executable-command-find-posix-p' in a non-installed tree actually
gives:
no such file or directory, /usr/local/share/emacs/21.3.50/site-lisp

As far as I know, `grep-find' previously worked fine with both POSIX
and `traditional' variants, e.g. from Solaris' /usr/bin and
/usr/xpg4/bin.  Now it looks as though it will fail other than with
GNU find even if something is done about the stuff above.

Please treat stuff from Aalto more circumspectly.

By the way, lisp/progmodes seems the wrong place for grep.el, and its
header line is wrong.