pki: Cache entered secrets in case they are needed more than once
pki: Load hmac plugin which is required to decrypt PKCS#12 containers
pki: Add simple PKCS#12 display command
pki: Reformat PKCS#12 output and add an index for each certificate/key
pki: Add command to export certificates and keys from PKCS#12 containers
pem: Handle BER indefinite length encoding as binary ASN.1

While our ASN.1 parser can't handle BER indefinite length encoding, the OpenSSL
backend can. Some PKCS#12 containers get encoded this way, so we should
support loading such files in the pem plugin.