upspin: turn the list of subcommands into a data structure
upspin: turn the list of subcommands into a data structure

This is easier to maintain and can produce a nicer error message.
No functional change.

Change-Id: If13b4d170889f01dd51329cbdfa29e3243e0645e
Reviewed-on: https://upspin-review.googlesource.com/2835
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin