setvbuf: Add support for disabling I/O buffering. Initially cut; untested.

include/nuttx/fs/fs.h

@@ -405,7 +405,7 @@ struct filelist
 struct file_struct
 {
   int                fs_fd;        /* File descriptor associated with stream */
-#if CONFIG_STDIO_BUFFER_SIZE > 0
+#ifndef CONFIG_STDIO_DISABLE_BUFFERING
   sem_t              fs_sem;       /* For thread safety */
   pid_t              fs_holder;    /* Holder of sem */
   int                fs_counts;    /* Number of times sem is held */

include/stdio.h

@@ -58,10 +58,12 @@
 
 #define FILENAME_MAX _POSIX_NAME_MAX
 
-/* The size of the I/O buffers */
+/* The (default) size of the I/O buffers */
 
-#if CONFIG_STDIO_BUFFER_SIZE > 0
-#  define BUFSIZ CONFIG_STDIO_BUFFER_SIZE
+#if defined(CONFIG_STDIO_BUFFER_SIZE) && CONFIG_STDIO_BUFFER_SIZE > 0
+#  define BUFSIZ   CONFIG_STDIO_BUFFER_SIZE
+#else
+#  define BUFSIZ   64
 #endif
 
 /* The following three definitions are for ANSI C, used by setvbuf */