ha: Double receive buffer size for HA messages and make it configurable

ha: Double receive buffer size for HA messages and make it configurable

With IKEv1 we transmit both public DH factors (used to derive the initial
IV) besides the shared secret.  So these messages could get significantly
larger than 1024 bytes, depending on the DH group (modp2048 just about
fits into it).  The new default of 2048 bytes should be fine up to modp4096
and for larger groups the buffer size may be increased (an error is
logged should this happen).