Add support for os exit logic

[elupus]

• Add support for os_exit() to allow a portable wait to indicate that system should shut down
• Add support for a delayed exit routine to indicate shutdown of system after a set delay

[elupus]

It's failing due to lack of support for long timers on linux. Will rebase once we have that fix in.

[CLAassistant]

All committers have signed the CLA.