telnet_server: increase buffer sizes to allow longer commands.

telnet_server: increase buffer sizes to allow longer commands.

A common use case seen in the wild is echoing a string of commands to an
existing openocd instance via netcat.  The sequence of ; separated
commands can easily run over the line limit of only 256 chars.

Increasing this dramatically reduces surprises, at the expense of a tiny
amount of extra ram usage.

Change-Id: I2389d99d316a96b5fa03f0894b43c412308e12c4
Signed-off-by: Karl Palsson <karlp@tweak.net.au>
Reviewed-on: http://openocd.zylin.com/4132
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>