dm cache policy mq: list args in alphabetical order

Alasdair advocated for this.. trivial.. and it enables the initial lvm2
support to be more intuitive (later lvm2 code will need to care about
arg order, when generating table lines for comparison, without trusting
alphabetical order anyway.. but whatever)
dm cache: eliminate extra space in status output before policy status is emitted
dm cache policy basic: list args in alphabetical order

again fluff, but not a big deal to have the initial code emit these
alphabetically
dm cache policy basic: set_config is no loner passed through to the policy
dm cache metadata: document the DM_CACHE_FEATURE flags
dm thin: document the DM_THIN_FEATURE flags

(also added DM_ prefix to the flags)
dm cache: remove set_config from message interface (from Heinz)
[dm-cache] typo in documentation
[dm-cache] tweak the status line