[system] align license and license banners with Fuchsia

This adjusts the Magenta userspace code to match the prevailing
license in Fuchsia (three-clause BSD), using the same banner
and license text.

Change-Id: I6b6d79435c5dbaa32027888874849e4f47aaa7be

[utest] Support categories

This will produce single binary from mxr tests and also seperate binary
for each test

Change-Id: I9345fbeab5751dda356cfd47a766c09957211923