Show what a desk's secret prompt is for, and declare mesh-mail's (issue 789)

The desk's prompt named only the module and the secret's key, so the operator
asking for one of mesh-mail's mail-credential-* accounts could not tell which
one. node-launcher.secret (modules/rofi) now shows the controller's optional
what as its own line under the module and secret names, escaped as text so
the catalogue's words never reach the prompt as markup; omitted as before
when none is declared. mesh-mail's mail-credential-* family now declares that
line with the long-form own-secrets shape the controller's manifest parser
reads (novox/hq issue 789 part b; part a landed in mesh-controller).

modules/mesh-mail/module.json

@@ -22,7 +22,8 @@
   "own-secrets": {
     "mail-credential-*": {
       "path": "${dir:state}/mail-credential-*.secret",
-      "issued-by": "outside"
+      "issued-by": "outside",
+      "prompt": "the password of the mail account {member}"
     }
   },
   "tools": [