nwdbm: honor ignore-case volumes when creating SYS dirs

Treat the `i` volume option like `k` when choosing the Unix-side
case for the automatically created SYS directories.

With a volume configured as `itO` (ignore case, trustees, OS/2 LFN),
the server currently recreates uppercase directories such as LOGIN,
PUBLIC, SYSTEM and MAIL on startup, even when the installed tree uses
lowercase login/public/system/mail. This leaves duplicate host
directories that appear as ambiguous entries from DOS clients.

Use lowercase Unix names for the auto-created SYS directories when
the volume is configured either as downshift (`k`) or ignore-case (`i`).

src/nwdbm.c

@@ -1963,7 +1963,16 @@ static int get_sys_unixname(uint8 *unixname, uint8 *sysname, uint8 *sysentry)
     if (founds > 2) {
       uint8 *p;
       for (p=optionstr; *p; p++) {
-        if (*p=='k') {
+        if (*p=='k' || *p=='i') {
+          /*
+           * Keep the automatically created SYS subdirectories in the
+           * canonical lower-case form when the volume is configured for
+           * either explicit downshift ('k') or case-insensitive lookup ('i').
+           * Without this, a SYS volume installed with lower-case directories
+           * such as public/login/system/mail and configured as 'itO' will
+           * recreate upper-case PUBLIC/LOGIN/SYSTEM/MAIL on every server
+           * start, which then shows duplicate DOS-visible entries.
+           */
           result|=1;  /* downshift */
         } else if (*p=='t') {
           result|=2;  /* trustees */