Cache the result of Zotlabs\Lib\System::get_platform_name

Hi,

I did some xhprof on hubzilla and found out that get_platform_name was a significant part of the CPU consumption. While the function is damn simple, its call count (way over 2000 for a single page) made caching it's result a low hanging fruit and a usefull thing to do. Their is probably many other things to do performnce wise, but nothing come as useful and as easy to do.

Patch attachedcache_plateform_name.patch