Issue 5839 - Spellchecker should avoid suggesting private symbols.

http://d.puremagic.com/issues/show_bug.cgi?id=5839

This is a **partial** pull.

@9rnsr: I've sent an e-mail to dmd-internals but it got bounced, so I'm CC'ing you. I want to define the various SEARCH flags because the `flags` parameter was never documented and the codebase has magic flags like `4|2`. I want to replace them with well-defined macros (similar to SCOPE macros). Do you know what each of them does?