Allow any type of private key in PEM files by treating PEM guards ending
with "PRIVATE KEY-----" as a private key instead of looking for a
pre-defined set of PEM guards. This enables the possibility for custom
GTlsBackend to add support for new key types.

Test cases have been expanded to ensure PEM parsing works for private
key when either header or footer is missing.

Encrypted PKCS#8 is still rejected. Test case has been added for this to
ensure behaviour is the same before and after this change.