Makefile: Do not install bash completion to /etc

`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>
Fix directory traversal for reencryption when $PREFIX is a symlink
Rephrase awkward/confusing message about enabling echo during password entry.
Use a glob in the pattern match instead of using two comparisons for [yY].