Allow HTTP protocol on unix local socket

Allow also Z39.50 database to be given for unix local socket.
The COMSTACK string for doing so is:
unix:<spec>:<uri>
Examples:
  unix:/myfile:tcp:localhost:9999/mybase
  unix:/myfile:localhost:9999/mybase
  unix:/myfile:http://fakehost/mybase
The fakehost is parsed in Host HTTP-header.