Let's add a dlopen_qrencode() function that does the actual dlopen()
stuff and caches the result.
This is useful so that we later can automatically test for all dlopen
hookups to work correctly.
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.