make installer: option to set address & port where server listened

Client SSH connections now create remote port forwarding port back
to the server, so that making the server listen only on "localhost"
provides an easy improvement for security (except that remote port
forwarding seems not to work with freeSSHd on Windows).
make installer: make Windows work with bash-serving sshd

Makes a Windows build client work with Cygwin's opensshd.
make installers: make the server listen only to "localhost" by default

This makes the build more secure, but it's a backward-incompatible change.
To adapt old site-configuration scripts, the simplest option is to set
`#:server' to "localhost" everywhere and rely on SSH tunneling to let
a client reach a server (though, unfortunately, that option doesn't
seem to work if a Windows machine uses freeSSHd). Another possibility
is to se `#:server-hosts' to the empty list.
Mac OS X: make updated frameowrk paths a multiple of 8

Needed for code-signing (on 64-bit builds, at least)
make installer: add Mac OS X code-signing support
make snapshot-site: add "current" links for installers