Several improvements required for <openssl/bn.h>:

Several improvements required for <openssl/bn.h>:
* List internal constants and types that are intentionally undocumented.
* List unused constants and types that are intentionally undocumented.
* Cope with intentionally undocumented identifiers being declared
more than once (in this case, because of #if and #else).
* Require exact matches for man -k searches (in this case,
such that BN_BITS does not match BN_BITS2).
* Handle the weird BN_ULONG, which is #define'd instead of using typedef.