Introduce new header file llappviewermacosx-for-objc.h to publish for
llappdelegate-objc.mm and other Objective-C++ consumers the free functions in
llappviewermacosx.cpp they consume. These were never before declared in any
header file. Apparently, to date, we've been trusting to luck that
Objective-C++ will infer the correct signature from calls -- and that the
calls are correct with respect to the function definitions.  :-P

This gives us a place to introduce a new getLogFilePathname() function to
query LLDir. (We don't simply #include "lldir.h" because of the pervasive use
of BOOL in viewer headers; BOOL means something very different in Objective-C++.)