Fix x509_extension event.

The event now really returns the extension. If openssl supports printing
it, it is converted into the openssl ascii output.

The output does not always look pretty because it can contain newlines.

New event syntax:
event x509_extension(c: connection, is_orig: bool, cert:X509, extension: X509_extension_info)

Example output for extension:
  [name=X509v3 Extended Key Usage,
    short_name=extendedKeyUsage,
    oid=2.5.29.37,
    critical=F,
    value=TLS Web Server Authentication, TLS Web Client Authentication]
  [name=X509v3 Certificate Policies,
   short_name=certificatePolicies,
   oid=2.5.29.32,
   critical=F,
   value=Policy: 1.3.6.1.4.1.6449.1.2.1.3.4^J  CPS: https://secure.comodo.com/CPS^J]

testing/btest/scripts/base/protocols/ssl/x509_extensions.test

@@ -0,0 +1,7 @@
+# @TEST-EXEC: bro -r $TRACES/tls1.2.trace %INPUT
+# @TEST-EXEC: btest-diff .stdout
+
+event x509_extension(c: connection, is_orig: bool, cert:X509, extension: X509_extension_info) 
+{
+	print extension;
+}