cmd/upspin: look up command name before setting config
cmd/upspin: look up command name before setting config

This allows for a much nicer error message (effectively the output of
upspin -help) if you mistype a command:

% upspin singup joe@blow.com
upspin: no such command "singup"
Upspin commands:
	shell (Interactive mode)
	countersign
	cp
	deletestorage
	deploy
	foo
	get
	getref
	info
	keygen
	link
	ls
	mkdir
	put
	repack
	rm
	rotate
	setupdomain
	setupserver
	setupstorage
	setupwriters
	share
	signup
	snapshot
	tar
	user
	whichaccess
%

Fixes #264.

Change-Id: I2425a214cdc96cbdf1588c66b039b85533497830
Reviewed-on: https://upspin-review.googlesource.com/8207
Reviewed-by: Eduardo Pinheiro <c9d6fd76b920b269b838bbb70f683c6db503d8d1@gmail.com>

upspin/upspin,upspin/upspin