Required according to RFC 3986:

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

Closes #330

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Eliminate dependency on io_thread_get().

Make sure we're using the same EventLoop as the NfsManager.

Eliminate dependency on io_thread_get().

The old struct config_param remains only for top-level string options.

Fixes hanging NFS client.

Avoids off-by-one bug and obsoletes the bug fix in commit 966c4244

More robust and cancellable.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

For strrchr().  Fixes build failure.