global: output usage on -h, --help, or no args before contacting mons

global: output usage on -h, --help, or no args before contacting mons

- when there are no arguments, print a short invitation to stderr to
  use -h or --help and exit with an error.
- if we get -h or --help, print usage to stdout, and exit with success.
- do the above *before* making any contact with the cluster.  we should
  not fail to explain usage because the mons are down.
- if there is some other error with the arguments, print an error message,
  but do not spam the user with usage.  Try to use cerr instead of derr.

Signed-off-by: Sage Weil <sage@redhat.com>