libalpm: add getter for handle on db and pkg

db and pkg store a pointer to the handle for internal use but don't
actually provide a way for a user to get it.

Making this accessible is more convenient for front ends and FFI

For example, in other languages it's common to return the error value
directly. To achieve this the python and rust wrappers also store their
own pointer to the handle inside their own pkg/db wrappers.

Exposing this would allow the wrappers to forgo the extra pointer and
just return `pkg.get_handle().last_error()`.
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