button handler w/o python

[Matt Spinler]

Hi,

In our quest to be able to remove python, I put up a review for a button
handler in C++ in phosphor-state-manager called phosphor-button-handler:
https://gerrit.openbmc-project.xyz/#/c/openbmc/phosphor-state-manager/+/15437/

It uses the signals sent by the phosphor-button code to support the 
power, ID,
and reset buttons, though it doesn't require them all to be present.

I realize systems may want to do their own unique things on button 
presses, but
I think this gives a good reference implementation.

Matt