nextcloud: repair only the module's own account, read its existence from the user list, no admin group (issue 1253, review 1)

occ's user:info says 'user not found' on stdout with an empty stderr, so the
error read as 'occ produced no output' and mesh-admin was never made on novox:
occ now says stderr and stdout both, and existence is read from user:list.
The repair could reset whatever account the user setting names, the
operator's included, and the 'admin' fallback is a hand-made install's human
administrator: only mesh-admin is repaired now, and it is the default. The
shares poll needs no admin rights, so the account gets no group. After a
repair the throttle of the caller's address is reset, and a failing repair
reaches the back-off's one loud line with occ's message.

modules/nextcloud/module.json

@@ -72,7 +72,7 @@
     },
     "user": {
       "type": "string",
-      "description": "The Nextcloud account the module's tools act as; empty is admin.",
+      "description": "The Nextcloud account the module's tools act as; empty is mesh-admin, the account the module makes and keeps for itself. Any other account is used as given and never repaired.",
       "trusted": true,
       "default": ""
     }