Merge pull request #4 from cjdoucette/u4id

Pull request to merge modifications to xip for the U4ID principal
Ordered principals' object files

See variable XIP_OBJ_INCLUDE in Makefile.
U4ID: proper sanitization of input

When a user calls 'xip u4id add 000...' with more than
XIA_MAX_STRID_SIZE digits, the variable static_xid would have
a non-terminated string.
U4ID: clarify help message

The ID non-terminal is different from others equivalent
non-terminal of other principals.
This patch just emphasizes it changing the name of it to UDP_ID.
U4ID: combine tunnel declaration into add command