prefer SUDO_USER over logname to find login for granting schroot access

SUDO_USER is reported to work more reliably in certain scenarios
(see #1713) and it can be overridden easily if running as root.
create the linux chroots under /var/chroot by default

This is a better location and more likely to have free disk space,
as /opt might be mounted with insufficient disk space (see #1713)
use gmake wherever it is available for the posix-local build

This is useful when the default make is not GNU make e.g on FreeBSD
update the installation instructions comprehensively