differential SETACL silently fails if no vfile exists

[Vincent Breitmoser]

In version 2.2.13, with `acl = vfile`, setting a differential ACL
permission doesn't work if there are no permissions set yet. Example
IMAP log:

b1 GETACL test
* ACL test valodim lrwstipekxacd
b1 OK Getacl completed.
b2 SETACL test valodim -l
b2 OK Setacl complete.
b3 GETACL test
* ACL test valodim lrwstipekxacd
b3 OK Getacl completed.

My expectation was that the file would be created with default
permissions, and the requested change then applied.  However, no acl
permission is set and no vfile is created for this mailbox in the
process.

This does work as expected (and a file is created) if I set a
non-differential ACL first:

b4 SETACL test valodim lrwstipekxacd
b4 OK Setacl complete.
b5 GETACL test
* ACL test valodim akxeilprwtscd
b5 OK Getacl completed.
b6 SETACL test valodim -l
b6 OK Setacl complete.
b7 GETACL test
* ACL test valodim akxeiprwtscd
b7 OK Getacl completed.

 - V