Pkcs12 support for --rpc-certificate

Pkcs12 allows to easily store and parse(!) certificate chains and keyfiles in a single file. Also easy enough to produce (openssl know a pkcs12 verb).

It seems that AppleTLS and my upcoming WinTLS (Schannel) implementations can implement .p12 far more easily than .pem (although I didn't try yet).

PEM is still supported in openssl and gnutls, of course.

Any objections?