modules/m_sasl.c: use IsSecure() instead of IsSSL()

modules/m_sasl.c: use IsSecure() instead of IsSSL()

Further to our implementation of the concept of "secure origins", we can
indicate to services that the client is connected securely, rather than
just that the client is using TLS. For example, connections from the
local host (from the IRCd's perspective) can be considered secure
against eavesdropping.

Allow this to factor into services' decision on whether to allow an SASL
negotiation or not. Atheme currently assumes this means the client is
using TLS, but I have changed that in atheme/atheme@412d50103c1d8c2a