Support for loading certificate chains in TLS context

lib/tlscontext.c: This modify the loading of certificate, to load a cert
chain intead of a single certificate. It don't break the earlier
funcionality, but extends it, if you do this:
cat ~/server.crt ~/signing.crt ~/intermediate.crt >/etc/syslog-ng/chain.crt
Assuming the following:
Other sides know a root.crt, which signed intermediate.crt, intermediate
signed signing.crt, and signing signed the server.crt.