Clean up/modernize Card and its subclasses a bit

In particular, add constructors for converting a Card to the specific
Card subclasses. This ensures that all state already set on a Card is
copied to the specific subclass when we construct it. Previously, we
copied the serial number (but no other state) manually.

Card:

• Document that Card represents one of possibly many applications on a smartcard
• Make setAppName() protected since it's only supposed to be called by the subclasses during initialization
• Remove unused (and obsolete) mSlot, slot(), setSlot()
• Initialize member variables in class definition

NetKeyCard, OpenPGPCard, PIVCard:

• Remove default constructors and unused constructors from serial number
• Add constructors for converting a Card to the specific Card subclasses