fix(health): give the LLDP phase a valid slot in the phase arrays

In HEALTH=1 builds health_phase(HEALTH_LLDP) used id 7 while ph_max[] and
ph_slow[] had HEALTH_PHASES = 6 entries. The LLDP checkpoint therefore
wrote one byte past ph_max (into the high byte of ph_slow[0]) and, when
the phase took two ticks or more, incremented two bytes past ph_slow
(dhcp_state.state and transaction_id on KP_9000_6XHML_X2).

Rename it HEALTH_PH_LLDP = 6, derive HEALTH_PHASES from it so the arrays
always cover the last phase, print an lldp line in "health" and document
it.

Fixes #658

CHANGELOG.md

@@ -25,6 +25,9 @@
 
 ## Fixed
 
+- Health
+  - `HEALTH=1` builds no longer write past the phase timing arrays at the LLDP checkpoint, which
+    corrupted the `link` slow counter and the DHCP client state. #658
 - Config
   - A startup configuration line longer than the command buffer no longer stops the replay:
     the offending line is skipped and the following lines are still applied.