apc_pub_encrypt: Let the callee allocate the buffer.

apc_pub_encrypt: Let the callee allocate the buffer.

Change the encryption routine of the apc API to allocate a suitably
sized buffer itself. Currently, the caller has to guess the size of the
buffer to pass to the function and we reuse our 4k handshake buffer for
that. While 4k is is plenty at the moment, that may change, and it's
always better to use the exact size if it is readily available. This
is the case here because the required buffer size is just the number
of bits of the modulus of the key.