Add detection for stdout being attached to a tty device. When this check
fails, return a default width of 0, which callers interpret to mean
"don't wrap". Conversely, when our term ioctl suceeds but returns 0, we
interpret this to mean a tty with an unknown width (e.g., a serial
console), in which case we default to a sane value of 80.
Signed-off-by: Dave Reisner <d@falconindy.com>