build: remove some unused stuff and clean up a bit

Remove some variables that are unused, but set. Also reorder and
comment some other sections.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
docs: add note about zsh and vim files

At least mention their existance in the INSTALL file.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>