Sentinel: INFO-CACHE reply format command shortened.
Sentinel: removed useless flag var from INFO-CACHE.
Sentinel: INFO-CACHE GCC minior code cleanup.

I guess the initial goal of the initialization was to suppress GCC
warning, but if we have to initialize, we can do it with the base-case
value instead of NULL which is never retained.
Sentinel: INFO-CACHE comments reworked a bit.

Changed in order to make them more review friendly, based on the
experience of reviewing the code myself.