SmartCard::ReaderStatus: prevent infinite loop caused by scdaemon re-writing reader_0.status on each SERIALNO command

We check the contents of the reader_*.status files, and only ping() the ReaderStatusThread
if they changed. Also, acknowledge that we can only deal with a single smartcard slot at
this time and simplify the code in the reader status thread accordingly.

svn path=/branches/kdepim/enterprise4/kdepim/; revision=1090927