Add tests for single quote with safe_shlex_split.
safe_shlex_split: Don't split None.
safe_shlex_split: Give up after 3 tries.
safe_shlex_split: Adjust docstring
safe_shlex_split: Fix broken \' in test.
safe_shlex_split: Use real lexer object and handle ' correctly.
safe_shlex_split: cleanup and don't do double-lexing