Fix for c832d464

`make install` should never install anything to /etc.
"/usr/share/bash-completion/completions/" is the right place for
completions that are installed by software packages.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>

The default permissions set by 'install' are 0755 which is not
what we want for manpages and the bash completion file.

This commit introduces these variables, and defaults to using
the same values as the ones that were used before. The advantage
of doing this is to make it easier to use some distro-specific
layout. When this is desirable, these variables can be overridden
to what is appropriate at package build time.