Add os_exec() helper to run external programs

Change-Id: I579af1fa8c2f85622ffddb186ba799dcb9ac4b6f
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
wpa_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: I0005ed08e4b06ba3d2ebe95b9240050e47ed2e8c
Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>