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.

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.

These have no use, exception for creating an artificial dependency
from input_plugins[] to all archive plugins.

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

Replace InputPlugin attributes.

Replaces the method Close().

Merge duplicate symbol.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.