Fix parsing of #ifdef in calendar files

There was code to process an #ifndef tokens, but none for #ifdef.
The #ifdef token was mentioned as unsupported in the BUGS section,
but no reason was given and I do not see why it should stay omitted.

Misleading information in The BUGS section of the man-page regarding
the maximum number of #define and #include statements supported has
been removed. These limits might have applied to a prior version of
this program, but do not seem to apply to the current implementation.

I have not tried to test for the existence of the limits, but the
include file processing just recursively calls the parser (without
counting the recursion depth) and the stringlist functions do not
impose a limit on the number of entries.

Reported by: jhs@berklix.com
MFC after: 3 days