pppd: Allow users to use Engine and ID strings unmodified to the OpenSSL engine ...

pppd: Allow users to use Engine and ID strings unmodified to the OpenSSL engine (#280)

This change allows users to specify "cert" and "key" arguments where the prefix
leading up to the ':' indicates the engine e.g. pkcs11, the entire string is
passed down to the engine library which does the parsing. In the case of
pkcs11.so (OpenSC project's libp11) engine for OpenSSL, it will now correctly
parse the PKCS11 URI string (See RFC7512).

Signed-off-by: Eivind Næss <eivnaes@yahoo.com>