A somewhat different approach to --enable-verbose: inline __wt_verbose()

@agorrod: Alex, I noticed we quite a few WT_UNUSED statements (and lint reports even more than gcc does, it complains when variables are written but never subsequently read).

Here's a slightly different approach for your consideration: I inlined the `__wt_verbose` call (inlined so the flag test wouldn't require a function call), and then all of the WT_UNUSED magic can go in one place, neither gcc or lint complain if you call a function and that function doesn't use the variables you pass it.

This also makes the verbose calls look like normal calls so we don't need the `WT_VERBOSE_{ERR,RET,TRET}` macros, we can fall back to the usual `WT_{ERR,RET,TRET}` calls.

This is a bigger change, but I think it's actually a simpler change and we'll avoid having to mark random variables as WT_UNUSED in the future.

One downside to this approach is we're inlining:
```
va_start(ap, fmt);
ret = __wt_eventv(session, 1, 0, NULL, 0, fmt, ap);
va_end(ap);
```
in every function that calls `__wt_verbose`.