id: -Zn/-Zr: avoid an invalid diagnostic

id: -Zn/-Zr: avoid an invalid diagnostic

* src/id.c (main): Using -Z with -r or -n would fail with "id: cannot
print only names or real IDs in default format", in spite of that "-Z",
which specifies a non-default format.  Now, it succeeds and ignores
the -n or -r option.  The error was that the test for default_format
was not updated when I added the new --context (-Z) option in
commit v6.9-33-g5320d0f.