libc/ftok: Map token to the root pseduo file system directory

to ensure the generated key is unique from each other since only
root pseduo file system really support st_ino field after:
commit d35fbf534d51f7ef72382d9666a1f19e07c6f00f
Author: Xiang Xiao <xiaoxiang@xiaomi.com>
Date:   Fri Sep 16 14:24:55 2022 +0800

    fs: Allocate unique serial number for the root pseduo file system node

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libc/misc/Kconfig

@@ -62,6 +62,12 @@ config LIBC_ENVPATH
 		Use the contents of the common environment variable to locate executable
 		or library files.  Default: n
 
+config LIBC_FTOK_VFS_PATH
+	string "Relative path to ftok storage"
+	default "/var/ftok"
+	---help---
+		The relative path to where ftok will exist in the root namespace.
+
 config LIBC_MEM_FD_VFS_PATH
 	string "Relative path to memfd storage"
 	default "memfd"

libs/libc/misc/lib_ftok.c

@@ -27,6 +27,8 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/ipc.h>
+#include <errno.h>
+#include <string.h>
 
 /****************************************************************************
  * Public Functions
@@ -56,11 +58,19 @@
 
 key_t ftok(FAR const char *pathname, int proj_id)
 {
+  char fullpath[PATH_MAX] = CONFIG_LIBC_FTOK_VFS_PATH "/";
   struct stat st;
 
-  if (stat(pathname, &st) < 0)
+  strlcat(fullpath, pathname, sizeof(fullpath));
+  if (stat(fullpath, &st) < 0 && get_errno() == ENOENT)
     {
-      return (key_t)-1;
+      /* Directory not exist, let's create one for caller */
+
+      mkdir(fullpath, S_IRWXU);
+      if (stat(fullpath, &st) < 0)
+        {
+          return (key_t)-1;
+        }
     }
 
   return ((key_t)proj_id << 24 |