Refactored Ip::Intercept::Lookup() (#1062)

Refactored Ip::Intercept::Lookup() (#1062)

The TPROXY lookup part was misleading -- the method only checked that
TPROXY was enabled by the configuration and did not change the accepted
connection settings.

Also do not assume that a listening port can have both COMM_TRANSPARENT
and COMM_INTERCEPTION flags set. Squid treats TPROXY and NAT modes as
mutually exclusive and rejects such configurations.

Also forbid invalid port configurations with tproxy mode where TPROXY is
prohibited by ./configure.

Also removed unused Ip::Intercept::StopInterception().