Fix one error and a host of warnings that are new to GCC 7 (currently in prerelease).
A few of the -Wimplicit-fallthrough= warnings are left, because I couldn't figure out
if they were actually intentional or not; someone else will need to look at those.

Change-Id: Iac43bec96e60a5f25f5a4d69319646de8cbe1e4f