Add spell checking for debug flags.

I just used `ninja -d stat` and it took me a bit to realize that I missed the
trailing 's'.

While here, move the message printing from printf() to Error(). This makes the
output consistent with other error outputs: The messages are now prefixed with
"ninja: error: " instead of just "ninja: " and they go to stderr instead of
stdout.
Merge pull request #522 from nico/spelldebug

Add spell checking for debug flags.