etcdmain: format usage

Fix #1649 

Follow the go cmd style and our doc at https://github.com/coreos/etcd/blob/master/Documentation/0.5/configuration.md

do we still want to print out the deprecated flags? 

/cc @bcwaldon @jonboulle @robszumski 
This might amount to a religious debate at this point, but we should really stick with the expected double-hyphen for mluti-character flags. I know golang allows both, but it's odd for non-golang folks.