mounts: name a malformed entry of sources by its position, never by its name

A password holding '=' and a space or comma splits the entry, and the tail
read as name=... was said as "the source <tail>", carrying part of the
secret into the health answer and the operator's conditions (review of
mesh-catalog#190, issue 386).

modules/mounts/README.md

@@ -70,8 +70,8 @@ Both are preferences whose default, `none`, names nothing (ADR 0262 allows no em
   source's username is in its entry: it is not a secret (novox/hq ADR 0283). It is percent-decoded, so a device that needs a domain takes it as
   `WORKGROUP%5Cuser` (for `WORKGROUP\user`), and an `@` in it is written `%40`; it holds no space, comma or
   `=`. A password never goes in the entry: `user:password@` is refused, saying so and never what was
-  written. An entry that cannot be read is said by its source's name, or by its position when even the name
-  cannot be read, and never with what it holds.
+  written. An entry that cannot be read whole is said as malformed by its position alone ("entry 2 of sources
+  is malformed: …"), never with its name or anything else it holds.
 
 A mount point is an absolute, plainly written path, not a directory of the system's own (`/etc`, `/usr`,
 `/run`, …). An entry that cannot be read is said, and only that entry is left out.