ArrayBuffer: fix copy-on-write for external data

ArrayBuffer: fix copy-on-write for external data

Previously, writes to ArrayBuffers returned by nginx body accessors
or raw exportKey() could modify the request body or key material.

Now, njs_vm_value_array_buffer_set() takes a shared argument, like
other creation APIs.  Shared data uses copy-on-write; private
WebCrypto results and joined fetch response bodies remain writable
without copying.

The issue was introduced in f6485009 (0.5.1).