Fix testsuite and bootstrap. Implement -I.

* bootstrap.conf: Include size_max.
* gnulib.modules: Remove memset, rmdir. Replace strdup with
strdup-posix. Patch by Eric Blake.
* src/tar.c: Implement -I as a shorthand for --use-compress-program.
* doc/tar.texi: Document -I.
* tests/pipe.at, tests/shortrec.at: Account for eventual 'Record
size' output.
* tests/testsuite.at (AT_TAR_CHECK_HOOK): New define
(AT_TAR_WITH_HOOK, TAR_IGNREC_HOOK): New macros.