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>
zsh: prevent warning message on old zsh.

Trying to determine whether a value is a number by using it in arithmetic
expressions makes zsh complain if it is not, in fact, a number.  Use grep to
check this instead, since it is more robust and less likely to complain.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
zsh: don't always print colors as bold.

Due to a bug in the color_fg function, colors in 16-color terminals using
zsh before 4.3.9 were always bold.  Fix this.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Merge branch 'master' into work