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

[Anders F Björklund]

Mark Farnell wrote:

> - Rather than letting the user to choose any directory, the installer
> can ask the user to choose whether it is for system or for user, if
> system, ask password and install in /opt/local/, if user, install in
> $(HOME)/opt/local

The major downside of a user install is that prefix paths are absolute,
which means that instead of /opt/local it will be /Users/afb/opt/local.
And since the archives and packages include this path, that means those
can't be used either... Unless they were already relocatable, that is.

--anders