slingshot: sync with upstream.

* slingshot: Sync with upsteram for grep GNUism fix.
* bootstrap.conf (buildreq): Don't set a git version number, or
else `GIT=true ./bootstrap` doesn't work!

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
zile: compatibility with introduction of posix.sys.

* lib/zile/getkey.lua (getkey): Allow for timeradd and timercmp
to be imported from the posix table (luaposix < 32) or from
the new posix.sys table (luaposix >= 32).

Signed-off-by: Gary V. Vaughan <gary@gnu.org>