add "configure" operation

[jcupitt]

This operation can be used to set various library config options, such
as vips_block_untrusted_set().

Having these options available as an operation makes like simpler for
bindings: they can expose them all via the usual introspection system.

Example:

pyvips.Image.configure(untrusted_block=True)

[jcupitt]

@kleisauke I thought this might be handy for bindings like NetVips. Rather than having to add API for every libvips configure option, we can just have a single vips operation which lets you set the config options in one place.

With this PR (for example), pyvips will instantly let you do:

pyvips.Image.configure(untrusted_block=True)

Same for lua-vips, ruby-vips, NetVips etc. And we can use the libvips arg system to safely deprecate, rename, add, extend, etc. options in future.

What do you think?

We should all the other config options too, eg concurrency, leak, etc. etc.

[jcupitt]

Notes:

• Maybe this should be called options? The name configure sounds too much like autotools and compile time configuration.
• We could allow read of settings as well as write.
• We could deprecate all the annoying vips_concurrency_set() etc. API.
• This adds GINT64 args, ie. signed int64s. I'm not sure all bindings support these, we should check.

[kleisauke]

This is interesting. It could prevent a lot of DllImport's in NetVips. For example, the Cache class might be rewritten to call the single Configure operation instead.

I had a quick attempt to fix the build and add this to NetVips, see:
kleisauke@43d2daf
kleisauke/net-vips@04b0aed

Reading settings would also be nice to have. In C# this would look like this:

NetVips.Image.Configure(cacheMax: 1000, cacheMaxFiles: 1000);
NetVips.Image.Configure(out var cacheMax, out var cacheMaxFiles);
Console.WriteLine(cacheMax); // 1000
Console.WriteLine(cacheMaxFiles); // 1000