enable installation check for driver.sys

For now we still only support subfunction 3 (return drive table) and subfunction 0 (check if installed) which we say yes.  This allows DOS Internals DRVINFO.EXE to correctly use internal drive table (it checks if installed and doesn't bother if not) and return corresponding information.  Note: subfunction 3 is still handled in int2f.asm for now, but may be moved to cleaner (but bigger and slower) implementation in inthndlr.c

kernel/int2f.asm

@@ -180,7 +180,7 @@ Int2f?iret:
 DriverSysCal:
                 extern  _Dyn
                 cmp     al, 3
-                jne     Int2f?iret
+                jne     IntDosCal ; pass other calls through
                 mov     ds, [cs:_DGROUP_]
                 mov     di, _Dyn+2
                 jmp     short Int2f?iret

kernel/inthndlr.c

@@ -2220,6 +2220,7 @@ VOID ASMCFUNC int2F_12_handler(struct int2f12regs FAR *pr)
 	  /* TODO */
       break;
 
+    /* Note: see in2f.asm where this is handled */
     case 0x03:                 /* get drive data table */
       r.DS = FP_SEG(&nul_dev);
 	  r.DI = FP_OFF(getddt(0));