Exit successfully when running a MAIN script with --help
Exit successfully when running a MAIN script with --help

The user requested help and got it, so it's fair to say that the request was
completed successfully. For comparison, I tested the following tools and all
of them exit with 0 when run with --help:
bash, gcc, perl, perl6, git, vim, ack, make, time, gdb, bc, cat

Fixes S06-other/main-usage.t

tbrowder/rakudo,jonathanstowe/rakudo,MasterDuke17/rakudo,nbrown/rakudo,sjn/rakudo,jonathanstowe/rakudo,MasterDuke17/rakudo,jonathanstowe/rakudo,salortiz/rakudo,usev6/rakudo,awwaiid/rakudo,MasterDuke17/rakudo,labster/rakudo,awwaiid/rakudo,tbrowder/rakudo,awwaiid/rakudo,salortiz/rakudo,usev6/rakudo,awwaiid/rakudo,MasterDuke17/rakudo,labster/rakudo,Gnouc/rakudo,salortiz/rakudo,softmoth/rakudo,rakudo/rakudo,tony-o/rakudo,raydiak/rakudo,salortiz/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,sjn/rakudo,sjn/rakudo,tony-o/rakudo,tony-o/rakudo,labster/rakudo,tony-o/rakudo,tbrowder/rakudo,usev6/rakudo,softmoth/rakudo,tbrowder/rakudo,tbrowder/rakudo,softmoth/rakudo,labster/rakudo,softmoth/rakudo,salortiz/rakudo,Gnouc/rakudo,lucasbuchala/rakudo,MasterDuke17/rakudo,jonathanstowe/rakudo,rakudo/rakudo,nbrown/rakudo,rakudo/rakudo,sjn/rakudo,Gnouc/rakudo,Gnouc/rakudo,softmoth/rakudo,salortiz/rakudo,lucasbuchala/rakudo,nbrown/rakudo,raydiak/rakudo,raydiak/rakudo,labster/rakudo,rakudo/rakudo,jonathanstowe/rakudo,lucasbuchala/rakudo,labster/rakudo,usev6/rakudo,raydiak/rakudo,awwaiid/rakudo,lucasbuchala/rakudo,nbrown/rakudo,usev6/rakudo,nbrown/rakudo,rakudo/rakudo,rakudo/rakudo,tony-o/rakudo,sjn/rakudo,nbrown/rakudo,MasterDuke17/rakudo,Gnouc/rakudo,tony-o/rakudo,tbrowder/rakudo