libc/stdio: Add stdio file locking functions

Add flockfile(), ftrylockfile() and funlockfile() functions [1].

[1] POSIX.1-2008 / System Interfaces / flockfile
    https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/functions/flockfile.html

Signed-off-by: Miguel Herranz <miguel@midokura.com>
Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libc/stdio/lib_libfflush.c

@@ -73,7 +73,7 @@ ssize_t lib_fflush(FAR FILE *stream, bool bforce)
 
   /* Make sure that we have exclusive access to the stream */
 
-  lib_take_lock(stream);
+  flockfile(stream);
 
   /* Check if there is an allocated I/O buffer */
 
@@ -156,11 +156,11 @@ ssize_t lib_fflush(FAR FILE *stream, bool bforce)
    * remaining in the buffer.
    */
 
-  lib_give_lock(stream);
+  funlockfile(stream);
   return stream->fs_bufpos - stream->fs_bufstart;
 
 errout_with_lock:
-  lib_give_lock(stream);
+  funlockfile(stream);
   return ret;
 
 #else