Reduce the default baud rate of PTY's to 9600.

Reduce the default baud rate of PTY's to 9600.

On RELENG_6 (and probably RELENG_7) we see our syscons windows and
pseudo-terminals have the following buffer sizes:

| LINE RAW CAN OUT IHIWT ILOWT OHWT LWT     COL STATE  SESS      PGID DISC
| ttyv0  0   0   0  7680  6720 2052 256       7 OCcl       1146  1146 term
| ttyp0  0   0   0  7680  6720 1296 256       0 OCc       82033 82033 term

These buffer sizes make no sense, because we often have much more output
than input, but I guess having higher input buffer sizes improves
guarantees of the system.

On MPSAFE TTY I just sent both the input and output buffer sizes to 7
KB, which is pretty big on a standard FreeBSD install with 8 syscons
windows and some PTY's. Reduce the baud rate to 9600 baud, which means
we now have the following buffer sizes:

|  LINE   INQ  CAN  LIN  LOW  OUTQ  USE  LOW   COL  SESS  PGID STATE
| ttyv0  1920    0    0  192  1984    0  199     7  2401  2401 Oil
| pts/0  1920    0    0  192  1984    0  199  5631  1305  2526 Oi

This is a lot smaller, but for pseudo-devices this should be good
enough. You need to do a lot of punching to fill up a 7.5 KB input
buffer. If it turns out things don't work out this way, we'll just
switch to 19200 baud.