Cleanup: add Security::ContextPointer as smart pointer to SSL_CTX*

Cleanup: add Security::ContextPointer as smart pointer to SSL_CTX*

Due to circular dependency issues between ssl/libsquidssl.la and
security/libsecurity.la the code within src/ssl/ is restricted to
only using Security::ContextPtr, it MUST NOT use ContextPointer

Code outside of src/ssl/ should always use Security::ContextPointer
when storing a reference to a context.

Unfortunately some uses of SSL_CTX_Pointer and AnyP::PortCfg remain
in src/ssl/support.cc for now.