scd rewrites the reader_0.status file with the same content whenever a client
issues a SERIALNO command, which, apparently GPA does to poll for a card, or
maybe just in response to the rewrite in the first place, triggering an endless
loop of 'rewrite' -> 'serialno' -> 'rewrite' ...
In any case, to avoid excessive filewatcher activity, and to break such cycles,
scd should check the current published state and only write the file if the
contents would change as a result.