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 policy mq: remove "promote threshold = 1, nr = 20" debugging
[dm-cache] typo in documentation
[dm-cache] tweak the status line
Merge branch 'thin-dev' of https://github.com/snitm/linux into thin-dev
dm cache policy mq: fix mq_status relative to threshold arguments

The table status must always reflect what the table was initialized
with.  emit_tunables() was always emitting the threshold arguments
regardless of whether they were specified during table load.

Remove descriptive context and arg count from STATUSTYPE_INFO; not aware
of other targets that emit arg counts, etc.- -- I don't really care if
we include "4 foo 3 bar 5" rather than just "3 5".. so feel free to
reintroduce this; but it _must_ not share the emit code with
STATUSTYPE_TABLE.

Introduce UNINIT_TUNABLE rather than using undocumented -1.
[dm-cache] redo messaging and status/table output.
Merge branch 'thin-dev' of https://github.com/snitm/linux into thin-dev

Conflicts:
	drivers/md/dm-cache-policy-mq.c
dm-cache-target: avoid promotion in case of writes == block_size
dm-cache-policy-background: add agressive write policy and support in dm-cache-policy-{basic,debug,mq}
Missing file
dm-cache-policy: introduce is_dirty method to avoid duplicating dirty block information in the background policy
dm-cache-policy-background: fix misplaced BUG_ON()
dm-cache-policy-basic: cleanup