Bugfix for:[23234] Blender File Browser - Back Button does work with first directory entered?

For !WIN32 systems the fix was in ED_fileselect_set_params
(basically adding the first folder in the sfile->folders_prev list)

For WIN32:
I talked with Nathan (Jesterking) and he agreed that the fix in path_util.c was required too. Without that BLI_path_abs was always making WIN32 paths ending with \ to end with \\
(e.g. C:\Blender\ --> C:\Blender\\)
And this was making the folder to fail ISDIR tests.

source/blender/blenlib/intern/path_util.c

@@ -637,6 +637,8 @@ int BLI_path_abs(char *path, const char *basepath)
 	
 	if (path[0]!='\0') {
 		if ( path[strlen(path)-1]=='/') {
+			/* remove the '/' so we avoid BLI_cleanup_dir adding an extra \ in WIN32 */
+			path[strlen(path)-1] = '\0';
 			BLI_cleanup_dir(NULL, path);
 		} else {
 			BLI_cleanup_file(NULL, path);

source/blender/editors/space_file/filesel.c

@@ -206,6 +206,13 @@ short ED_fileselect_set_params(SpaceFile *sfile)
 		params->filter = 0;
 		params->sort = FILE_SORT_ALPHA;
 	}
+
+
+	/* initialize the list with previous folders */
+	if (!sfile->folders_prev)
+		sfile->folders_prev = folderlist_new();
+	folderlist_pushdir(sfile->folders_prev, sfile->params->dir);
+
 	return 1;
 }