DIR: don't display invalid data with devices

[andrewbird]

When doing DIR NUL findfirst will actually return success with an attribute indicating a device exists with that name. MS-DOS shows 'File not found' in this case, but FreeCOM (and also Comcom64) can display invalid data, so let's correct that. Since the required constant FA_DEVICE isn't always present in dos.h, let's define it when necessary.

[#181]

Before (C: is MFS, D: is FAT16):

C:\>dir nul
 Volume in drive C is IR DXXXXS C
 Directory of C:\

NUL                      0  10-30-25  6:55p
         1 file(s)              0 bytes
         0 dir(s)     49,839 Mega bytes free
C:\>d:

D:\>dir nul
 Volume in drive D has no label
 Volume Serial Number is 4A09-DBB4

 Directory of D:\

NUL                      0  10-30-25  6:55p
         1 file(s)              0 bytes
         0 dir(s)      42,698,752 bytes free

With patch (C: is MFS, D: is FAT16):

C:\>dir nul
 Volume in drive C is IR DXXXXS C
 Directory of C:\

File not found.

C:\>d:

D:\>dir nul
 Volume in drive D has no label
 Volume Serial Number is 4A09-DBB4
 Directory of D:\

File not found.