datastore should complain if no args

[emiltin]

[info] load names from: ""
[info] size: 0
[info] allocating shared memory of 1031 bytes
[info] all data loaded

[springmeyer]

+1. I also ran osrm-datastore with no args the first time expecting to get usage. I then realized I needed -h but the first run had already lead to shared memory being loaded, which was clearly not valid shared memory.

[DennisOSRM]

Does now complain:

$ ./osrm-datastore 
[warn] no parameters given. try
./osrm-datastore --help

[springmeyer]

what about just showing the usage right off? That is a command behavior I see.

[DennisOSRM]

@springmeyer coming with #951