Renamed LLCurl::check_curl_code() and LLCurl::check_curl_multi_code() to prevent ambiguous name build error.

 * Removed check_curl_code and check_curl_multi_code from the global namespace.
 * Added comments documenting which thread the public methods of LLProxy should be called from.
 * Corrected grammar in LLSingleton.h
 * Fixed a buffer scope problem in llpacketring.cpp.

Clarified the reason for adding the deleteSingleton method to LLSingleton. Added a simple unit test to verify the functionality of the deleteSingleton method.

improved graphics and layout for logged in mode selector

 EXP-1079 FIX "User is not logged out immediately inworld when selecting to change modes and restart while logged in"