Small stylistic changes by VM:
- is_enabled() -> isenabled()
- static ... Py_<func> -> static ... gc_<func>

debug_cycle(), and don't cast the pointer to a long.  Neither needs
the literal `0x' prefix as %p automatically inserts this (on Linux at
least).

so we get better error messages.

Change a cast, intialize a local, and make some sprintf() format strings
type-appropriate (add the "l" to "%d").

Closes SourceForge patch #100737.

conditionally in the code.