Fix LLUI::locateSkin() failure case; clarify lldir.cpp static init.

Per code review:
Previous refactoring of LLUI::locateSkin() preserved odd failure behavior: it
would return last-considered pathname, whether or not it exists. Changed to
emit LL_WARNS log message and return empty string.
Use Boost.Assign to simplify initialization of a couple static containers in
lldir.cpp.