Clarify CMS_decrypt behaviour.
Extension checking fixes.

When looking for an extension we need to set the last found
position to -1 to properly search all extensions.

PR#3309.
Document -debug_decrypt option.
Fix SSL_CTX_get{first,next}_certificate.

Fix typo that stopped SSL_CTX_get_{first,next}_certificate from working.
Merge pull request #2 from openssl/master

pull updates from origin master