Use `id -u' to see if we're running as root,

Use `id -u' to see if we're running as root,
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.