Added mail directory argument to folder_addmsg in order to make it possible

Added mail directory argument to folder_addmsg in order to make it possible
to provide a path to the ext_hook call that is mailpath-based.  A problem
existed when a folder was a symbolic link and the pwd call would return
the path relative to the filesystem, not to mailpath.  A new argument was
needed because there was otherwise no reasonable way to get that path.