AUDIT: Avoid log pollution by untrusted strings.

We log strings from userspace, such as arguments to open(). These could
be formatted to contain \n followed by fake audit log entries. Provide
a function for logging such strings, which gives a hex dump when the
string contains anything but basic printable ASCII characters. Use it
for logging filenames.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>