Generalize framework to register monitoring of files in nscd

Generalize framework to register monitoring of files in nscd

nscd can clear caches when certain files change.  The list of files
was hardcoded so far and worked for nss_files and nss_dns and those
modules which need no monitoring.  nss_db, for instance, has its
own set of files to monitor.  Now the NSS modules themselves can
request that certain files are monitored.