Changeset 18040d3 in rtems for c/src/lib

Patch from Thomas Doerfler <td@…> to add flow control:

Some lines for "documentation":
======================================
One thing should be noted: when XON/XOFF is enabled, the serial
device will always work with one-character buffers, so the interrupt
load for the CPU might get higer, especially on devices like MC68360
and MPC860, where the serial channels are capable of using big
buffers. But, once again, this only happens when XON/XOFF is actually
selected.

Please note that the flag IXON is set by default, so outgoing
XON/XOFF flow control is enabled by default.

XON/XOFF is controlled using the "standard" fields IXON/IXOFF in the
termios structure. The termios flag IXANY is not (yet) supported.

Hardware handshake for the incoming data stream is controlled using
the standard flag CRTSCTS. If this flag is set, whenever the receive
buffer is almost full, the driver function "device.stopRemoteTx()" is
called, when the receive buffer has more space available,
"device.startRemoteTx()" is called again. If the driver does not
provide these interface functions (entries in device structure are
NULL pointers), then these calls are suppressed.

Changes of the flow control options during operation should work at
any time, but this has not been extensively tested.

No changes to the device driver interface are needed.
================================================

One critical point when using this patch might be, that any BSP using
this version of termios will now have outgoing flow control enabled
by default, so the behaviour of these BSPs will change here. The
option IXON has already been set in older termios by default, but it
did not work until this patch. Maybe this option should be switched
off by default, what do you think?