install.sh: use DESTDIR like variables for basedir, webroot and cgiroot

install.sh: use DESTDIR like variables for basedir, webroot and cgiroot

In preparation for a more atomic installation where the new versions of
$cfg_basedir, $cfg_webroot and $cfg_cgiroot are created in another location
and then renamed into place, create three new variables basedir, webroot
and cgiroot and switch to them instead of cfg_basedir, cfg_webroot and
cfg_cgiroot.

We must be careful though to only switch the places that are actually
creating or modifying installation files and not the places that are writing
configuration data or printing advice.

This change is strictly a variable name replacement (with a few added lines
to initialize the new variables) and does not change the installation
process at all.  That is left to the next step.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>