Provide CardOS 5 with its own decipher routine

Similar in essence to iso7816_decipher(), but uses extended APDUs (needed for
4096-bit keys) and restores a leading zero byte stripped by the card, a
behaviour which seems impossible to circumvent: the card either strips the
entire PKCS#1 BT 2 padding, or just the leading zero.
Support arbitrary key sizes in pkcs15_prkey_decrypt()

Allocate 'decrypted' according to the RSA key modulus length. Fixes
decription with 4096-bit keys.
Support large curves; from @szikora -- thanks!