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 fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

Eliminate dependency on io_thread_get().

Got lost in commit 6ed77f2a

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

Add enum InputResult which is a tri-state.  Input plugins may now fail
and just become unavailable.

Merge duplicate code.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.

Not used by any plugin currently, but this eliminates the g_error()
call in input_plugin_config(), so it's worth it.