missing: don't try to re-run tar with a munged command line

missing: don't try to re-run tar with a munged command line

* lib/missing: If the default `tar' program fails with the given
arguments, and GNU tar is not available, don't try to re-run the
default `tar' with a munged command line (e.g., ditching possibly
unportable options), as that could be subtly alter the intended
semantics (and maybe even create a somewhat corrupted tarball).
Also, it's worth noting that the main purpose of the `missing'
script is to allow a non-developer to build the package in the
face of slightly-skewed timestamps, not to provide wrappers for
all the maintainer tools -- so we don't have to try too hard
when `missing' is just called to wrap `tar'.
* tests/missing-tar.test: New test.
* tests/Makefile.am (TESTS): Add it.