Restore ability for MsgBase.open() to open an arbitrary SMB msgbase
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.