Added Makefile for installing libxia and xip

The Makefile at the root directory of this repository invokes the
Makefiles in libxia/ and xip/, and also knows how to install them.
This commit also removes the howto file.
Merge pull request #3 from abreen/master

Add Makefile for installing libxia and xip

The Makefile at the root directory of this repository invokes the
Makefiles in libxia/ and xip/, and also knows how to install them.

This commit also removes the howto file.
xip/ppk: add a '\n' at the end of an error message
principals: add zFilter XID type

Also rename 4id to i4id to make names consistent with
our documents.
Add xip u4id
Remove routes from U4ID