Description of problem:

POSIX's <limits.h> requires MAX_INPUT to be defined correctly and be at least
_POSIX_MAX_INPUT i.e. 255 chars. For the termios drivers, this means the serial
input buffer needs to be at least 255 chars - a CDL option for strict
conformance should exist which sets this.

However for the value of MAX_INPUT, it requires a call to serial_get_config with
CYG_IO_GET_CONFIG_SERIAL_BUFFER_INFO to get the buffer size and thus the value
of MAX_INPUT, so MAX_INPUT will have to be a sysconf()'d value... which means
implementing sysconf()!