Distinguish disabled from removed backend state; add make fixstyle

Add StateDisabled for operator-initiated disable, keeping StateRemoved
for backends that disappear during a config reload. Previously both
used StateRemoved, which was confusing: "removed" implies the backend
no longer exists in config, but a disabled backend is still present
and can be re-enabled on the fly.

- health: add StateDisabled with String() "disabled", Disable() method
  with probe code "disabled". Record() rejects probes in all three
  inactive states (paused, disabled, removed).
- checker: DisableBackend calls backend.Disable() instead of Remove().
- docs: healthchecks.md rewritten for pause (goroutine cancelled, not
  just results discarded), and separate disabled/removed state rows.
  user-guide.md updated to match.
- Makefile: add fixstyle target (gofmt -w .).

tests/01-maglevd/01-healthcheck.robot

@@ -59,7 +59,7 @@ Resume backend restarts probing
 
 Disable backend stops probing
     Maglevc    set backend nginx2 disable
-    Backend Should Have State    nginx2    removed
+    Backend Should Have State    nginx2    disabled
     Backend Should Be Disabled    nginx2
     Sleep    1s
     ${before} =    Get Probe Count    nginx2