Before commit 5da26eca, you could pass Msgbase() the path to an SMB
on the disk and open() it, no configuration needed. As of 2 years
ago, I broke that, and passing a path to an SMB would open the "mail" base
instead - most unexpected. This is a feature of smb_open_sub() which we
switched to using (from smb_open()), so go back to using smb_open() when
an unrecognized code is pass to the constructor.

This has the negative consequence that the "mail" base can't be created
via JS. Probably should fix that.