Support printing children search scope

This fixes 2caeef4.
Have positive and negative cache timeouts

The positive value determines the time a found entry is valid, the
negative timeout determines the lifetime of not found entries.
Implement a cache configuration option

This adds the cache nslcd.conf configuration option to configure the
dn2uid cache in nslcd with a positive and negative cache lifetime.
Use dn2uid cache options

The configuration values are used in the cache to determine positive and
negative hit TTLs. This also allows completely disabling the cache.
Document cache option in manual page
Add a test for new configuration option
Make dn2uid cache tuneable

This introduces a new cache configuration option that allows setting
positive and negative cache lifetimes for the dn2uid cache.