zshrc: use xo instead of co for terminal testing.

On some systems, the co (termcap columns) field is not present in the
appropriate terminfo file, leading to inaccurate messages about invalid
capabilities and broken prompt handling.  Fix this by using xo (termcap
XON/XOFF flow control) instead, which seems to work correctly on those
systems and others.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>