Fix aan on AMD64 and solve endianness issues.

The sizeof(Hdr) returns 16 on AMD64, instead of 12,
because of structure padding.

This problem made aan to fail early when establishing
a new connection.

The data exchanged over the network were endian
dependant because of the absence of proper
structure marshalling. We have chosen to specify
them as little-endian to keep backward compatibility
with old aan running on x86 and arm platforms.

Also remove the cosmetic t_Buf in the meantime.

This change was originally suggested by Erik Quanstrom.