Remove the dual-callback scheme for numeric and pointer thread IDs,

Remove the dual-callback scheme for numeric and pointer thread IDs,
deprecate the original (numeric-only) scheme, and replace with the
CRYPTO_THREADID object. This hides the platform-specifics and should reduce
the possibility for programming errors (where failing to explicitly check
both thread ID forms could create subtle, platform-specific bugs).

Thanks to Bodo, for invaluable review and feedback.