daemon: Install 'authenticate' script under LIBEXECDIR/guix.

daemon: Install 'authenticate' script under LIBEXECDIR/guix.

That way it is handled in the same way as other helper scripts.

* nix/scripts/guix-authenticate.in: Rename to...
* nix/scripts/authenticate.in: ... this.
* config-daemon.ac: Adjust accordingly.
* nix/local.mk (libstore_a_CPPFLAGS): Remove -DOPENSSL_PATH.
(nodist_libexec_SCRIPTS): Remove.
(nodist_pkglibexec_SCRIPTS): New variable.
* nix/nix-daemon/guix-daemon.cc (main): Remove 'setenv' call for
"PATH".
* nix/libstore/local-store.cc (runAuthenticationProgram): New function.
(LocalStore::exportPath, LocalStore::importPath): Use it instead of
'runProgram' and OPENSSL_PATH.