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 Comcom
(and also FreeCOM) can display invalid data, so let's
correct that. Since the required constant _A_DEVICE isn't
always present in `dos.h`, let's define it when necessary.

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

2025-10-30 16:41             0 NUL
         1 file(s)              0 bytes
         0 dir(s)            48.7 GB free
~~~

With patch:
~~~
C:\>dir nul
 Volume in drive c is IR DXXXXS C
 Directory of c:\

File not found
~~~

Author: andrewbird

src/command.c

@@ -50,6 +50,9 @@
  */
 
 #include <dos.h>
+#ifndef _A_DEVICE
+#define _A_DEVICE 0x40u
+#endif
 #include <libc/dosio.h>
 #include <io.h>
 #include <libc/getdinfo.h>
@@ -2693,7 +2696,8 @@ static void perform_dir(const char *arg)
       }
     if (first)
       {
-      if ((ffrc = findfirst_f(full_filespec, &ff, attrib, &ffhandle)) != 0)
+      if (((ffrc = findfirst_f(full_filespec, &ff, attrib, &ffhandle)) != 0) ||
+          ff.ff_attrib == _A_DEVICE )
         {
         puts("File not found");  // informational message -- not an error
         return;