libunit-wasm: Add a luw_mem_splice_file() function

This is inspired by the likes of splice(2) and sendfile(2) in that it
takes data from one place and puts it in another.

This function write(2)'s the request data straight from the shared
memory to a given file (referenced by its file descriptor).

This is an alternative to using luw_req_buf_copy() and avoids an extra
copying of the request data.

E.g

  /* In the request_handler */
  if (total_bytes_wrote == 0) {
          luw_init_ctx(&ctx, addr, 0);
          luw_set_req_buf(&ctx, &request_buf, LUW_SRB_NONE);

          fd = open("/var/tmp/large-file.dat", O_CREAT|O_TRUNC|O_WRONLY,
                    0666);
  }

  total_bytes_wrote += luw_mem_splice_file(addr, fd);
  if (total_bytes_wrote == luw_get_http_content_len(&ctx)) {
          close(fd);
          total_bytes_wrote = 0;
          luw_http_response_end();
  }

NOTE:

We include a typedef definition for ssize_t in unit-wasm.h, to avoid
having a dependency on the wasi-sysroot when generating the rust
bindings.

ssize_t is defined in sys/types.h which is provided by libc and not the
compiler.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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