command-line containing --outformat is less-error prone.

command-line containing --outformat is less-error prone.

Since --convert isn't mandatory anymore for --outformat to be used
(what was conflicting with --mutt-query), we removed the call to
set_convert_var() in 141acfcc.

We now need to test that one of MODE_CONVERT and MODE_QUERY is 'on'
before allowing the use of --outformat.
$ abook --outformat anyformat < file	# won't hang anymore