`(exit N); exit N'. Otherwise, those many tests could exit with
improper exit status when exiting via e.g., a trapped interrupt.
Thanks to a report from Bob Proulx.
rather than trying go write to an write-protected file.
When running as root, ensure $NON_ROOT_USERNAME is valid.
When running as root with `require-non-root', ensure that `.'
is writable by $NON_ROOT_USERNAME, then reinvoke $0 set-user-ID
to $NON_ROOT_USERNAME. If `.' is not writable, then skip the test.
