Add full support for initing registers.

- Fixes a few bugs in untested functionality.
- Add tests for the way the register handling code is used.
- Fix a few tests that were not reaping child processes.

Bug: 23762183

Test: Ran unit tests on host (32 bit and 64 bit).
Test: Ran unit tests on angler (32 bit and 64 bit).
Change-Id: I573d6617b4f1561f6e8494d7213c52086d112d97