typed arrays: copy Buffer in typed array constructor

Convert the Buffer to an ArrayBuffer. The typed_array.buffer property
should be an ArrayBuffer to avoid confusion: a Buffer doesn't have a
byteLength property and more importantly, its slice() method works
subtly different.

That means that before this commit:

  var buf = new Buffer(1);
  var arr = new Int8Array(buf);
  assert.equal(arr.buffer, buf);
  assert(arr.buffer instanceof Buffer);

And now:

  var buf = new Buffer(1);
  var arr = new Int8Array(buf);
  assert.notEqual(arr.buffer, buf);
  assert(arr.buffer instanceof ArrayBuffer);
doc: update buffer/typed array documentation

Clarify that typed array constructors accept buffers as their first
argument but that the memory is not shared and why this was changed
in v0.9.
test: fix common.mustCall

Don't run the 'has function been called?' checks if the test is exiting
with an error because a failed check will mask the real exception.

v0.8 doesn't have the _fatalException machinery in src/node.js and
src/node.cc so it doesn't have this issue.