[$] watch_mount(), watch_sb(), and fsinfo() (again)

Filesystems, by design, hide a lot of complexity from users. At times,though, those users need to be able to look inside the black box and extractinformation about what is going on within a filesystem. Answering thisneed is David Howells, the creator of a number of filesystem-orientedsystem calls; in thispatch set he tries to add three more, one of which we have seen beforeand two of which are new.