Have the configure script arrange that the Makefile define _U_

Have the configure script arrange that the Makefile define _U_
appropriately, and that GNUmakefile and the MSVC++ project file define
it apppriately, as we do with libpcap, rather than defining it in
"interface.h".

Undo the rcsid-shuffling and addition of extra #includes, as we no
longer need to arrange that "interface.h" be included before using _U_
in an RCS ID or copyright.