mounts: test the clearing loop, the look's own clearing and a disk's presence; clear only a taken-over unit's earlier failure; clear and re-arm an automount that gave up; refuse a transient unit of the same name (review of #162)

modules/mounts/README.md

@@ -48,7 +48,10 @@ off, or a node's share whose server is away, tried when someone opened the mount
 carried before the module wrote it (the unit fstab's generator makes until the line is adopted). The module
 says such a share as `absent` or `unreachable`; left failed, the unit would also be said as a failed unit of
 the machine's own, with a notification each time it is touched. A mount that failed while its source answers
-is left failed: that is a fault, and the share is said `failed`.
+is left failed: that is a fault, and the share is said `failed`. An automount that gave up after repeated
+failed mounts (`mount-start-limit-hit`) while its source was away is cleared and armed again the same way.
+Only a unit the module takes over has its earlier failure cleared regardless of the source; a pair of its
+own that it rewrites keeps its failure.
 
 A share is read-only when the setting says `:ro` or the sharing machine exports it read-only. A
 read-write share is mounted hard, so a write is never dropped while the server is away.
@@ -110,6 +113,8 @@ Each refusal is said as the share's state `failed`, with why, and nothing is wri
 - a mount point the system owns (`/`, `/etc`, `/usr`, `/boot`, `/proc`, `/sys`, `/dev`, `/run` and what
   is under them, and `/var`, `/home`, `/root`, `/tmp` themselves);
 - a mount point that something else is mounted at, or an automount that is not the module's;
+- a mount point whose unit someone made at run time (`/run/systemd/transient`, from `systemd-mount` or the
+  like): that is somebody's mount now;
 - a folder that holds files: a mount would hide them. Only a folder not in the kernel's mount table is
   opened, so a server that is away cannot hold the look;
 - a share that this machine serves itself.