[netstack] always close the zx_socket

Rationalize expected errors:

- loop{Control,Read} don't need to check for ZX_ERR_BAD_HANDLE
- zxwait.Wait never returns ZX_ERR_PEER_CLOSED
- zx_socket_{read,write} never return ZX_ERR_CANCELED

NET-2068 #done

Test: fx run-image-test netstack_loopback_test \
    --gtest_filter=NetStreamTest.BlockingAcceptWriteMultiple \
    --gtest_repeat=1000
Change-Id: I03fc862152cac22804d2c53422f75e1f5afd046a