Cleanup/refactor: Add new BLI_string_util.

Things like `BLI_uniquename` had nothing, but really nothing to do in
BLI_path_util files!

Also, got rid of length limitation in `BLI_uniquename_cb`, we can use
alloca here to avoid overhead of malloc while keeping free size (within
reasonable limits of course).

source/blender/blenlib/BLI_string_utils.h

@@ -0,0 +1,58 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2017 by the Blender Foundation.
+ * All rights reserved.
+ *
+ * The Original Code is: all of this file.
+ *
+ * Contributor(s): none yet.
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+#ifndef __BLI_STRING_UTILS_H__
+#define __BLI_STRING_UTILS_H__
+
+/** \file BLI_string_utils.h
+ *  \ingroup bli
+ */
+
+#include <stdarg.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "BLI_compiler_attrs.h"
+
+struct ListBase;
+
+typedef bool (*UniquenameCheckCallback)(void *arg, const char *name);
+
+int BLI_split_name_num(char *left, int *nr, const char *name, const char delim);
+
+bool BLI_uniquename_cb(
+        UniquenameCheckCallback unique_check, void *arg, const char *defname, char delim, char *name, size_t name_len);
+bool BLI_uniquename(
+        struct ListBase *list, void *vlink, const char *defname, char delim, int name_offs, size_t len);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  /* __BLI_STRING_UTILS_H__ */