int2f: Allow 1217h function to return new CDS entry

DOS provides the interrupt 2f/12xx functions for use by
filesystem redirectors in order that they do not need to access
structures and variables using undocumented methods. Interrupt
2f/1217 is a function used by a redirector to get the CDS entry
corresponding to a drive without iterating the CDS array found
via SYSVARS LOL.
    FreeDOS currently validates the flags in CDS entry before
deciding whether to return the entry to the caller. This behaviour
prevents a redirector receiving the CDS entry corresponding to
an as yet unassigned drive so preventing any new drive mapping.

    I've tested the following flavours of DOS and only FreeDOS
does this.

 # DOS variants returning new CDS entries okay
    DR-DOS
        6.00-930319 7.00 7.01 7.02-971119 7.02-980123 7.03 8.00
    MS-DOS
        3.10 3.20 3.21 3.30-Nec 3.30 3.31 4.01 5.00 6.00 6.20 6.21 6.22
        7.00 7.10
    PC-DOS
        3.00-Compaq 3.00 3.10-850307 3.10-850422 3.10-Compaq 3.20-851230
        3.20-860221 3.30 3.31-Compaq 4.00 4.01 5.00 5.02 6.10 6.30 7.00
        7.10 7.2K

 # DOS variants that return error rather than new CDS entry
    FR-DOS
        1.00 1.10 1.20

    This patch introduces an unvalidated version of get_cds() and calls
it only for the int2f/1217h call, other uses remain as before.

kernel/dosfns.c

@@ -93,6 +93,13 @@ STATIC int remote_lock_unlock(sft FAR *sftp,    /* SFT for file */
                              unsigned long len, /* length (in bytes) of region to lock or unlock */
                              int unlock);       /* one to unlock; zero to lock */
 
+struct cds FAR *get_cds_unvalidated(unsigned drive)
+{
+  if (drive >= lastdrive)
+    return NULL;
+  return &CDSp[drive];
+}
+
 /* get current directory structure for drive
    return NULL if the CDS is not valid or the
    drive is not within range */

kernel/inthndlr.c

@@ -1917,7 +1917,7 @@ VOID ASMCFUNC int2F_12_handler(struct int2f12regs r)
                                    ; probable use: get sizeof(CDSentry)
                                  */
       {
-        struct cds FAR *cdsp = get_cds(r.callerARG1 & 0xff);
+        struct cds FAR *cdsp = get_cds_unvalidated(r.callerARG1 & 0xff);
 
         if (cdsp == NULL)
         {

kernel/proto.h

@@ -118,6 +118,7 @@ COUNT DosLockUnlock(COUNT hndl, LONG pos, LONG len, COUNT unlock);
 int idx_to_sft_(int SftIndex);
 sft FAR *idx_to_sft(int SftIndex);
 int get_sft_idx(UCOUNT hndl);
+struct cds FAR *get_cds_unvalidated(unsigned dsk);
 struct cds FAR *get_cds(unsigned dsk);
 struct cds FAR *get_cds1(unsigned dsk);
 COUNT DosTruename(const char FAR * src, char FAR * dest);