* dcrt0.cc (dummy_autoload): Add load statement for RegDeleteValueA.

* external.cc (get_cygdrive_prefixes): New function.
(cygwin_internal): Add CW_GET_CYGDRIVE_PREFIXES case.
* path.cc (mount_info::read_cygdrive_info_from_registry): Read system cygdrive
prefix if user one is undefined.
(mount_info::write_cygdrive_info_to_registry): Write cygdrive prefix to the
appropriate registry hive.  Overwrite in-memory copy of cygdrive, if
appropriate.
(mount_info::remove_cygdrive_info_from_registry): New method.
(mount_info::get_cygdrive_prefixes): New method.
(cygwin_umount): Remove cygdrive prefix, if appropriate.
* registry.cc (reg_key::killvalue): New method.
* shared.h (class reg_key): Add killvalue, remove_cygdrive_info_to_registry,
and get_cygdrive_prefixes declarations.
* include/sys/cygwin.h (cygwin_getinfo_types): Add CW_GET_CYGDRIVE_PREFIXES.

winsup/cygwin/registry.cc

@@ -171,6 +171,17 @@ reg_key::kill (const char *name)
   return RegDeleteKeyA (key, name);
 }
 
+/* Delete the value specified by name of current key.  Returns the error code
+   from the RegDeleteValueA invocation. */
+
+int
+reg_key::killvalue (const char *name)
+{
+  if (key_is_invalid)
+    return key_is_invalid;
+  return RegDeleteValueA (key, name);
+}
+
 reg_key::~reg_key ()
 {
   if (!key_is_invalid)