Display build metadata on `siad` startup and via `siac version`

Created by: tbenz9

When running your own build it can sometimes be difficult to identify exactly which commit you are running from. This adds additional build information to the siac version command, and displays the git revision hash when siad starts up. If the build was not completed via the Makefile siad will display a warning on start and siac version will not display any of the build information.