tests: use chroot --user rather than internal setuidgid tool

tests: use chroot --user rather than internal setuidgid tool

* init.cfg (require_root_): Adjust to use chroot, and make
`require_built_ chroot` implicit when chroot used in the test.
* po/POTFILES.in: Remove reference to setuidgid tool.
* src/.gitignore: Likewise.
* src/local.mk: Likewise.
* src/setuidgid.c: Remove.
* tests/cp/preserve-gid.sh: s/setuidgid/chroot --user/.
* tests/cp/special-bits.sh: Likewise.
* tests/id/setgid.sh: Likewise.
* tests/misc/truncate-owned-by-other.sh
* tests/mv/sticky-to-xpart.sh: Likewise.
* tests/rm/fail-2eperm.sh: Likewise.
* tests/rm/no-give-up.sh: Likewise.
* tests/touch/now-owned-by-other.sh: Likewise.
* tests/misc/chroot-fail.sh: Skip if chroot not built.