varlinkctl: do not clobber format flags in verb_call

Currently, when SD_JSON_FORMAT_OFF is set in verb_call, the json format
flags are set to SD_JSON_FORMAT_PRETTY_AUTO|SD_JSON_FORMAT_COLOR_AUTO,
rather than or'ing those flags in. This means that other flags that may
have been set, e.g. SD_JSON_FORMAT_SEQ when --more is set, will be
clobbered.

Fix this by masking SD_JSON_FORMAT_OFF out, and then or'ing the new
flags in.