[SETUPLIB][USETUP] Move IsValidPath() back into setuplib for reusability (#7186)

Reverts the IsValidPath() move done in commit 9c64b57dc.

- Turn IsValidPath() into a IsValidInstallDirectory() helper function
  available in the setuplib, so that it can also be used in the GUI setup.

- Introduce a IS_VALID_INSTALL_PATH_CHAR() macro.

base/setup/lib/setuplib.c

@@ -712,6 +712,51 @@ InitSystemPartition(
     return TRUE;
 }
 
+BOOLEAN
+IsValidInstallDirectory(
+    _In_ PCWSTR InstallDir)
+{
+    UINT i, Length;
+
+    Length = wcslen(InstallDir);
+
+    // TODO: Add check for 8.3 too.
+
+    /* Path must be at least 2 characters long */
+//    if (Length < 2)
+//        return FALSE;
+
+    /* Path must start with a backslash */
+//    if (InstallDir[0] != L'\\')
+//        return FALSE;
+
+    /* Path must not end with a backslash */
+    if (InstallDir[Length - 1] == L'\\')
+        return FALSE;
+
+    /* Path must not contain whitespace characters */
+    for (i = 0; i < Length; i++)
+    {
+        if (iswspace(InstallDir[i]))
+            return FALSE;
+    }
+
+    /* Path component must not end with a dot */
+    for (i = 0; i < Length; i++)
+    {
+        if (InstallDir[i] == L'\\' && i > 0)
+        {
+            if (InstallDir[i - 1] == L'.')
+                return FALSE;
+        }
+    }
+
+    if (InstallDir[Length - 1] == L'.')
+        return FALSE;
+
+    return TRUE;
+}
+
 NTSTATUS
 InitDestinationPaths(
     IN OUT PUSETUP_DATA pSetupData,