Optimize cache enable/disable and update_mod_rewrite_rules

[stodorovic]

• Don't show htaccess form if cache is disabled. It doesn't make sense to update rewrite rules in this case.
• Don't get test pages if $add_rules is false.
• Cleanup/Optimize wp_cache_enable, wp_cache_disable, wp_super_cache_enable, wp_super_cache_disable.
• Remove rewrite rules if caching is disabled.

[donnchawp]

Thanks again! You're really digging into some old code here.

Those calls to wp_cache_replace_line() could even be replaced with calls to wp_cache_setting() now.

And this proves how old some of this code is, I'm pretty sure WordPress doesn't have a gzipcompression option any more!

[stodorovic]

It'll be possible after merging #487. I think that current wp_cache_setting doesn't support boolean.