According to the crash reports, it's possible for the grammar checking
thread to call GetGrammarChecker, instantiating lightproof, at the same
time as the main thread instantiates LngSvcMgr, which also instantiates
(some?) (all?) grammar checkers.

Ensure that pyuno_loader::CreateInstance() initialises Python only
once with a C++11 thread safe static.

Change-Id: I5b1faba9107355c508831a078366e4a29fdbfadf