upbox: support starting a cacheserver for any user
upbox: support starting a cacheserver for any user

Upbox will now start a cacheserver for a specific user when instructed.
For example:

	users:
	- name: joe
	  cache: true
	servers:
	...

Change-Id: I921a3808c0627b1747be49cac15c92567721bfa8
Reviewed-on: https://upspin-review.googlesource.com/13664
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin