Add support for Unix domain socket connection

[horus]

As discussed in #95, this commit brings back the support for Unix domain socket connectivity.

The parser in readHostPortM now works this way:

• It attempts to parse the input as hostname or hostname:port (the old behavior),
• in non-windows systems, if above failed, check whether hostname equals "unix", then take the rest part as the path to the socket file.

If someone really named their host unix together with a all-digits file name in the current working directory, then he/she can always specify the path like unix:./8080 to avoid being parsed as a hostname-port pair.