hooks/common.sh: Rename installCentOS4Package to installRPMPackage

Add installCentOS4Package wrapper for backward compatibility.
Factor out functions to test if we should use yum or apt
isAPT: Also check for dpkg and rename it to isDeb

That way we should not try to use Debian/Ubuntu package names if
apt4rpm is available.
Fix filesystem tools installation in 91-install-fs-tools

Was broken since 4.3rc1.

Merged 91-install-fs-tools back into 90-make-fstab. Added support for
RPM-based distributions, too, so it's more generic now.
Factor out legacy grub menu.lst generation
hooks/common.sh: installDebianPackage no more installs recommends
hooks/common.sh: Fix syntax error in installCentOS4Package wrapper.
Better check for grub-legacy availability which ignores virtual packages
Remove trailing blank line
Call update-grub at the end of 82-install-grub-legacy to update menu.lst
chomp the output of dpkg --print-architecture
Create /boot/grub/default if non-existent

Seems to be needed at least for Squeeze.