transform: split out transform-common.h

To avoid too much duplication, some boilerplate and helpful code from
transport.c is now brought out into macros in transport-common.h.

It will be reused in the later commits introducing the EAX transform.

Also, rename transform.c to transform-cbcmac.c, etc.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>