cmd/stgenfiles: Add utility for generating test data
cmd/stgenfiles: Add utility for generating test data

I use this to generate ~40 gigs of random test data in 200k files:

    ~/stgenfiles -dir /data/benchdata -files 200000 -maxexp 22 -src /dev/urandom

canton7/syncthing,syncthing/syncthing,kamadak/syncthing,uok/syncthing,AudriusButkevicius/syncthing,ProactiveServices/syncthing,brgmnn/syncthing,kluppy/syncthing,imsodin/syncthing,kluppy/syncthing,jryans/syncthing,brgmnn/syncthing,plouj/syncthing,TheBigBear/syncthing,norgeous/syncthing,cherylquirion/syncthing,alex2108/syncthing,toddleish/syncthing,ProactiveServices/syncthing,letiemble/syncthing,uok/syncthing,canton7/syncthing,wweich/syncthing,syncthing/syncthing,kamadak/syncthing,nrm21/syncthing,brgmnn/syncthing,kamadak/syncthing,syncthing/syncthing,TheBigBear/syncthing,Kudalufi/syncthing,lkwg82/syncthing,nrm21/syncthing,imsodin/syncthing,le9i0nx/syncthing,AudriusButkevicius/syncthing,xduugu/syncthing,cherylquirion/syncthing,TheBigBear/syncthing,imsodin/syncthing,alex2108/syncthing,le9i0nx/syncthing,TheBigBear/syncthing,uok/syncthing,syncthing/syncthing,kamadak/syncthing,letiemble/syncthing,jryans/syncthing,plouj/syncthing,canton7/syncthing,lkwg82/syncthing,syncthing/syncthing,ProactiveServices/syncthing,xduugu/syncthing,cherylquirion/syncthing,nrm21/syncthing,toddleish/syncthing,toddleish/syncthing,alex2108/syncthing,imsodin/syncthing,uok/syncthing,le9i0nx/syncthing,xduugu/syncthing,wweich/syncthing,plouj/syncthing,nrm21/syncthing,toddleish/syncthing,Kudalufi/syncthing,syncthing/syncthing,rumpelsepp/syncthing,plouj/syncthing,cherylquirion/syncthing,brgmnn/syncthing,Kudalufi/syncthing,lkwg82/syncthing,wweich/syncthing,letiemble/syncthing,jryans/syncthing,letiemble/syncthing,le9i0nx/syncthing,xduugu/syncthing,norgeous/syncthing,alex2108/syncthing,AudriusButkevicius/syncthing,wweich/syncthing,Kudalufi/syncthing,norgeous/syncthing,rumpelsepp/syncthing,norgeous/syncthing,rumpelsepp/syncthing,jryans/syncthing,AudriusButkevicius/syncthing,rumpelsepp/syncthing,imsodin/syncthing,kluppy/syncthing,kluppy/syncthing,canton7/syncthing,lkwg82/syncthing,ProactiveServices/syncthing