[VM] virtual folders inheriting readonly

[Julian Bradfield]

One thing that has annoyed me for many years is that when you make a
read-only folder, and then create a virtual folder by selection from
it, the resulting folder is not read only.
(This is annoying because you then end up changing attributes on
messages that you had just explicitly tried to avoid changing, by
setting readonly in the first place.)

So it seems to me that virtual folders by selection should inherit
readonly from their parents, even if it's not explicitly specified.

Against 8.1.1, this is

--- vm-virtual.el 2011/11/25 21:51:36 1.1
+++ vm-virtual.el 2011/11/25 21:56:10 1.2
@@ -269,6 +269,7 @@
(nconc (vm-read-virtual-selector "Create virtual folder of
messages: ")
(list prefix))))
(vm-select-folder-buffer)
+ (if vm-folder-read-only (setq read-only t))
(vm-check-for-killed-summary)
(vm-error-if-folder-empty)
(let ((use-marks (eq last-command 'vm-next-command-uses-marks))