ws: Ignore SIGCHLD in mock-sshd tests

Trying to solve Travis failures here, and this may be the cause
of the /web-service/fail-spawn/rfc6455 and similar tests failing.

```
/bin/sh: 1: /nonexistant: not found
[2014/08/10 16:52:22.192502, 3] ssh_socket_unbuffered_write:  Enabling POLLOUT for socket
[2014/08/10 16:52:22.237944, 3] packet_send2:  packet: wrote [len=28,padding=13,comp=14,payload=14]
[2014/08/10 16:52:22.238731, 1] channel_write_common:  channel_write wrote 5 bytes
[2014/08/10 16:52:22.239538, 3] packet_send2:  packet: wrote [len=76,padding=10,comp=65,payload=65]
[2014/08/10 16:52:22.239662, 1] channel_write_common:  channel_write wrote 56 bytes
[2014/08/10 16:52:22.239991, 3] ssh_packet_socket_callback:  packet: read type 93 [len=28,padding=18,comp=9,payload=9]
[2014/08/10 16:52:22.240110, 3] ssh_packet_process:  Dispatching handler for packet type 93
[2014/08/10 16:52:22.240228, 2] channel_rcv_change_window:  Adding 1248005 bytes to channel (43:43) (from 31939 bytes)
[2014/08/10 16:52:22.240399, 3] ssh_socket_unbuffered_write:  Enabling POLLOUT for socket
[2014/08/10 16:52:22.241281, 1] ssh_socket_exception_callback:  Socket exception callback: 1 (0)
[2014/08/10 16:52:22.241734, 1] ssh_socket_exception_callback:  Socket error: disconnected
**
cockpit-ws:ERROR:src/ws/test-webservice.c:409:expect_control_message: assertion failed (json_object_get_string_member (options, expect_option) == expect_value): ("terminated" == "no-agent")
FAIL: test-webservice 19 /web-service/fail-spawn/rfc6455
```