ODS2: add linking to files / directories

[Rhialto]

As noted in open-simh/simh#215 (comment)
it might be useful to have an option in ods2 to do the equivalent of

$ mount dua1: STANDBACKUP
$ set def dua1:[000000]
$ set file/enter=[000000]sysexe.dir [sys0]sysexe.dir

to fix up bootable disks.