Bug 2741 (partial): Initial libsecurity API

The first step(s) towards a generic TLS/SSL security API for Squid.

Creates the basic security/libsecurity.la library and Security::
namespace infrastructure. Symbols provided by this API are always
available instead of conditionally compiled (unlike the ssl/* code for
OpenSSL use).

Merge the TLS/SSL context parameters into a Security::PeerOptions
object instead of maintaining multiple member variables in the
CachePeer and SquidConfig objects.

Squid now provides an error if SSL-specific squid.conf parameters are
used for a Squid without OpenSSL support, instead of silently ignoring
them.