Refactor + reorganize openssl files

- All streams-related code now lives in xp_ssl.c. Previously stream
  code was split across both openssl.c and xp_ssl.c
- Folded superfluous php_openssl_structs.h into xp_ssl.c
- Misc refactoring
Merge remote-tracking branch 'rdlowrey/reorg' into reorg
Merge remote-tracking branch 'rdlowrey/reorg' into reorg
More xp_ssl.c refactoring