Modify module_tool tar extract for non-gnu tar

This commit refactors how tar is called when the module tool extracts a
downloaded archive so that it is compatible with generic Unix tar, and
does not require GNU extensions. The commit allows the module tool to
work out of the box on Solaris 10, without requiring access to the gtar
command, and presumably this aspect of the module tool will now work
with tar on other old-style unix platforms as well.