The install_java.sh script was not setting the appropriate java and
javac as default. This is not an issue if no other JDK is installed, but
if so it could cause the wrong JDK to be used. So it now sets the
appropriate default system JDK.
Also, added a comment at the top of the install_java.sh and
install_java.bat files indicating it configures the default system
JDK and informs the user how to go about selecting a different
default JDK if necessary.
Do not download 7zip if it had previously been downloaded, for example
if this script had previously been run. And don't delete it at the end
of this script in case this script is run again. This is an attempt to
save time and bandwidth.
Rather than having a separate install_java.sh script for distributions
that use a different package manger, I updated install_java.sh to
handle package managers used by various Linux distributions.
