Installer should allow installation of macport in custom locations - for users without admin access

[Mark Farnell]

It is nice to see that everything is now contained in /opt/local/ and
/usr/ is no longer touched.

However to allow macport to be installed by users without admin
privilege within their home directory, we should allow users to
specify custom install directory, and if the user has write access to
the destination directory, user should *not* be asked for password.

Mark