Found with modernize-use-override
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Behave like STL.

Just like std::condition_variable, which however has no way to specify
the std::mutex directly.

Closes #477

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Since nfs_stat_async is deprecated.

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

Fixes two build failures with libnfs on Windows.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate dependency on io_thread_get().

The Connect method can be called between Schedule and lock. In that case, when
locked, the state is already set to CONNECTING of READY and the condition won't
be signaled anymore.

Return false on error, not nullptr.