Added HTTP tunnel facility for COMSTACK, bug #1752.

This is a facility that allows a Web proxy, such as squid, to tunnel
Z39.50 traffic. This facility is "transparent" to must applications
using YAZ. It's enabled by using pseudo transport connect: followed bý
the we proxy address, followed by command, then follwed by he regular
"virtual" addresss. For example,
connect:webproxy.com:3128,tcp:z3950.loc.gov:7090/voyager .