crypt: Rename RSA functions.

This renames the functions and constants of the public crypto API
which deal with RSA to have the common "apc" (asymmetric pubkey
cryptosystem) prefix.  This hides RSA as an implementation detail
and makes it clear that the functions/constants are related to apc.

Pure renaming, no semantic change. The only other changes are a new
comment to crypt.h and a \ref statement in the doxygen comment of
apc_free_pubkey().