install: don't use makeself

Instead, use a base64 installer. This allows to be more flexible. We
can modify the file and $0 don't have to be correct. The script could
be slightly modified to be run as a `.bashrc` for example.
install: fallback to "openssl base64" if needed
install: also try python for base64 decode
zshrc: don't overwrite ZSH if already set
install: don't use exit, use return.
install: how to use it as a .bashrc