Low: libstonithd: check libcrmservice status rather than exit code for timeouts

Previously, libstonithd checked the svc_action_t rc (agent exit status) for
PCMK_OCF_TIMEOUT to detect timeouts. However, this logically is an execution
status, so check status for PCMK_EXEC_TIMEOUT instead.

This should not have any effect since the code currently always sets the two
values together.