fusedav segfaults if group name specified in run_as_gid does not exist

[joemiller]

Ran into this during COS work. It was very difficult to debug this issue. Led to a few days of time spent with trial and error and guessing at different settings. Not critical, probably, but would be good to try to fix this for our future selves.

To reproduce, set run_as_gid foo, and ensure there is no group named foo in the host or the container's /etc/group file . fusedav will segfault on startup without any useful log message as to why.

It should handle this gracefully. Ideally it would say "unknown group 'foo'" and exit with an error code.