node: fix removing AsyncListener in callback

context._asyncQueue shouldn't be exposed as asyncQueue, as it allows
modification of queues already attached to an event. Which is not
supposed to happend. Instead context._asyncQueue should be copied.
v8: unbreak freebsd build

Pending review of https://codereview.appspot.com/31500043/

Fixes #6576.
util: Format negative zero as '-0'

Format negative zero as '-0' instead of as '0', as it does not behave
identically to positive zero. ((-0).toString() still returns '0' as
required by ES5 9.8.1.2).

Fixes joyent/node#6548.
Closes joyent/node#6550.
Merge pull request #13 from joyent/master

sync