Add part of chain verify SSL support code: not complete or doing anything

yet.

Add a function X509_STORE_CTX_purpose_inherit() which implements the logic
of "inheriting" purpose and trust from a parent structure and using a default:
this will be used in the SSL code and possibly future S/MIME.

Partial documentation of the 'verify' utility. Still need to document how all
the extension checking works and the various error messages.