alter cabal-install bootstrap.sh to not download the extra package
tarballs if these tarballs are already present.
allow cabal-install bootstrap.sh without user package database

This changes how the SCOPE_OF_INSTALLATION variable is used to provide
an alternate way of running bootstrap.sh which can install
cabal-install to a separate prefix and package database without
altering the user's package database or using it. The idea with this
is to improve the repeatability of automatically installing ghc, cabal
and other haskell independently of the current standard installation
or the user's main package database.

Usage example:

ghc-pkg init /home/jake/test_cabal/
SCOPE_OF_INSTALLATION='--global --package-db=/home/jake/test_cabal/packagedb' PREFIX=/home/jake/test_cabal/prefix ./bootstrap.sh

If you don't override the SCOPE_OF_INSTALLATION variable, bootstrap.sh
works as before.
bootstrap.sh: add comments on SCOPE_OF_INSTALLATION for custom package
database install
bootstrap.sh: fix the comment for SCOPE_OF_INSTALLATION
Merge pull request #2133 from JakeWheat/bootstrap-tweaks

Bootstrap tweaks