Allow custom baud rates for FTDI serial ports

- Termios: cf{get,set}{i,o}speed can handle arbitrary speed values.
- The value is stored in the appropriate fields of the termios structure
  in this case. The old constants (stored in the flags) are preserved
  for BeOS binary compatibility.
- Adjust the FTDI FT232* driver to accept custom rates, by replacing the
  hardcoded regster values with a function that will compute it
  according to FTDI documentation (confirmed giving the same values for
  the existing baudrates).

headers/posix/termios.h

@@ -21,8 +21,8 @@ struct termios {
 	tcflag_t	c_cflag;	/* control modes */
 	tcflag_t	c_lflag;	/* local modes */
 	char		c_line;		/* line discipline */
-	speed_t		c_ispeed;	/* (unused) */
-	speed_t		c_ospeed;	/* (unused) */
+	speed_t		c_ispeed;	/* custom input baudrate */
+	speed_t		c_ospeed;	/* custom output baudrate */
 	cc_t		c_cc[NCCS];	/* control characters */
 };