DIR: don't display invalid data with devices
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.
[https://github.com/FDOS/freecom/issues/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.
~~~