Deprecate "sg run-set", use "sg start" instead, rework help output

Created by: mrnugget

Alright, this does two things:

• Deprecates sg run-set, because it's a confusing name. Instead we made sg start to everything that run-set did: sg start, without arguments, starts the default environment. sg start <commandset> starts the given commandset.
• Rework and fix the sg -help and sg <subcommand> -help output. Now it actually helps users.

And when I say "deprecate" I mean: breaking change. run-set is gone. It breaks. sg start is the new thing.

Update: it's deprecated, not broken. It'll print a message.