MINOR: proxy: introduce proxy_abrt_close_def() to pass the desired default

MINOR: proxy: introduce proxy_abrt_close_def() to pass the desired default

With this function we can now pass the desired default value for the
abortonclose option when neither the option nor its opposite were set.
Let's also take this opportunity for using it directly from the HTTP
analyser since there's no point in re-checking the proxy's mode there.