DIR U12 wildcard support

[dansanderson]

DIR U12 lists the files in the current working directory of the SD card. Currently, when U12 is specified, other arguments like a filename pattern are silently ignored: DIR "PAT*",U12 also lists all files. It would be nice if DIR U12 supported a wildcard pattern argument. SD card listings get very long in typical uses, and limiting the files that come back would be useful.

Because U12 is not a DOS device, this will have to be implemented differently to how DOS implements wildcards. Ideally they'd share an implementation and feature set, but it might be easier to offer limited pattern support with just the * at first.

Pagination should continue to be supported, e.g.: DIR "PAT*",P,U12