#3 Check return codes everywhere

[Markus Elfring]

Some checks for return codes are missing.

Example:
Please add more error handling for return values from
"pthread_mutex_lock" in the functions like
"process_update" and "cache_known_bad".
http://cvs.sourceforge.net/viewcvs.py/cpcd/cpcd/src/cpcd.c?rev=1.9&view=markup