Make crypo a per fd feature

Make crypo a per fd feature

The current code checked if the global pointers crypt_function_recv and
crypt_function_send are non-NULL. In this case it used the given crypt
functions unconditionally for each fd. This makes it messy to enable
crypto only for some fds.

This patch removes these global pointers in favour of an array of
crypt function pointers. Users may use this function to activate
crypto for each fd separately.