Allow Updating Keys and Certificates in a KeyChain

[cfis]

The goal of this patch is to enable updating keys and certificates stored in a key chain. It does this by moving access to SecItemUpdate to the Sec::Base class. This was inspired by the section “Adding, Removing, and Working With Keys and Certificates” in the Key Services Programming Guide which mentions SecItemAdd, SecItemUpdate and SecItemCopyMatching as base functions that should be used on keys and certificates stored in the KeyChain. This patch does make it possible for example to change a key’s name as stored in the keychain (my particular use case).

[cfis]

Any chance this could get merged?