Add trap_GetNews UI system call

[zturtleman]

Tremulous' UI's qboolean trap_GetNews(qboolean begin) which downloads tremulous.net/clientnews.txt to news.dat and puts it in cl_newsString.

Add int trap_GetNews(const char *url, char *buffer, int bufferSize, qboolean begin) to UI. Download URL to modname/news.dat, save URL to modname/news_url.dat. Add status codes for return. Status' meaning: cURL not enabled at compile time, unable to load cURL library, loaded cached news, retrieving file, ...