journal,homectl: unify implementations of libqrencode loading and fss key printing

journal,homectl: unify implementations of libqrencode loading and fss key printing

We had two of each: both homectl and journalctl had the whole dlopen()
wrapper, and journalctl had two implementations (slightly different) of the
code to print the fss:// pattern.

print_qrcode() now returns -EOPNOTSUPP when compiled with qrcode support. Both
callers ignore the return value, so this changes nothing.

No functional change.