[handles] Tweak zx_handle_close_many's behavior on ZX_HANDLE_INVALID

Also tweaks the tests and adds comments. In particular, make it clear
that iteration over duplicate handles still succeeds in closing all
handles.

Test: modified handle-close test

Change-Id: If89046260473d5f45a768388665f859e7ae9c8fb

[handles] Introduce zx_handle_close_many

Test: handle-close-test tests success and failure cases.

Change-Id: I85bd0fc52aad474e826a4ec8c6e3ee7f94f039b0