journal-remote: convert to parse_boolean_argument() and fix type confusion

journal-remote: convert to parse_boolean_argument() and fix type confusion

We were passing a reference to 'int arg_seal' to config_parse_bool(),
which expects a 'bool *'. Luckily, this would work, because 'bool'
is smaller than 'int', so config_parse_bool() would set the least-significant
byte of arg_seal. At least I think so. But let's use consistent types ;)

Also, modernize style a bit and don't use integers in boolean context.