In macOS, when building an X509 chain, if a leaf and issuing certificate
form a chain, but the subject and authority key identifiers do not match,
an IdLinkage status is reported.

To match Windows, we ignore this chain status.

Linux appears to use these extensions when building a chain, and reports
PartialChain even if the attributes were not strictly needed to aid in
building a chain.

Additionally, this updates the debugging output for macOS to use an allocating
function to convert a CFString to a C string. CFStringGetCStringPtr will return
"NULL if the internal storage of theString does not allow this to be returned efficiently."
which it appears to have started doing for me.