Re-implement comment parsing missing in the internal pre-processor

The internal pre-processor ignored lines that did not parse a calendar
entries, but did not support multi-line comments in the way the external
cpp did.

The calendar files distributed with the base system (now in a port) do
use comments, though.

Implement comment processing for single-line (//) and multi-line comments
(/* */) with same semantics as in a standard C pre-processor.

All tests pass with this version, but there are no tests that specifically
verify comment processing.

Reported by: jhs@berklix.com (Julian H. Stacey)
MFC after: 3 days