r3443: the next stage in the include files re-organisation.

r3443: the next stage in the include files re-organisation.

I have created the include/system/ directory, which will contain the
wrappers for the system includes for logical subsystems. So far I have
created include/system/kerberos.h and include/system/network.h, which
contain all the system includes for kerberos code and networking code.
These are the included in subsystems that need kerberos or networking
respectively.

Note that this method avoids the mess of #ifdef HAVE_XXX_H in every C
file, instead each C module includes the include/system/XXX.h file for
the logical system support it needs, and the details are kept isolated
in include/system/

This patch also creates a "struct ipv4_addr" which replaces "struct
in_addr" in our code. That avoids every C file needing to import all
the system networking headers.