Also:
* do some light refactoring of create-client/node
* make it clear that these commands' --basedir options do the same as
the global --node-directory option
* use "global-options" instead of "global-opts"
The new rules for "bin/tahoe ARG1.. SUBCOMMAND ARG2.." arg:
* --node-directory is only accepted in ARG1, not ARG2
* create-*/start/stop/restart accept --basedir in ARG2, or an explicit
basedir argument
* only one of --node-directory/--basedir/explicit-basedir is accepted
* --quiet/--version is only accepted in ARG1, not ARG2
Closes#166
