libgloss: fix read and write prototype

Use _READ_WRITE_RETURN_TYPE for read/write in libgloss/{read,write}.c to
avoid mismatching types.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>

libgloss/read.c

@@ -20,7 +20,7 @@ extern char inbyte (void);
  * read  -- read bytes from the serial port. Ignore fd, since
  *          we only have stdin.
  */
-int
+_READ_WRITE_RETURN_TYPE
 read (int fd,
        void *buf_,
        size_t nbytes)

libgloss/write.c

@@ -21,7 +21,7 @@ extern int  outbyte (char x);
  *          stdout and stderr are the same. Since we have no filesystem,
  *          open will only return an error.
  */
-int
+_READ_WRITE_RETURN_TYPE
 write (int fd,
        const void *buf_,
        size_t nbytes)