audit: audit_string_contains_control can be boolean

This patch makes audit_string_contains_control return bool to improve
readability due to this particular function only using either one or
zero as its return value.

Signed-off-by: Yaowei Bai <bywxiaobai@163.com>
[PM: tweaked subject line]
Signed-off-by: Paul Moore <pmoore@redhat.com>