cmd/cacheserver: Accept -cachedir flag again
cmd/cacheserver: Accept -cachedir flag again

The cacheserver instances that were normally started by cmd/upspin and
cmd/upspinfs were complaining about -cachedir not being defined.

Change-Id: I2e27716737ab7dd425afd5308f8fe24fb1c70ea1
Reviewed-on: https://upspin-review.googlesource.com/9041
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

upspin/upspin,upspin/upspin