hostapd_cli: Use os_exec() for action script execution

Use os_exec() to run the action script operations to avoid undesired
command line processing for control interface event strings. Previously,
it could have been possible for some of the event strings to include
unsanitized data which is not suitable for system() use. (CVE-2014-3686)

Change-Id: If46d6cfcb9d7fc9700965e818315e5aa50fa11a5
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>