zshrc: support PAGER='less -Mr' usage and fall back to vi if LESSOPEN is unset

zshrc: support PAGER='less -Mr' usage and fall back to vi if LESSOPEN is unset

It's horribly annoying when PAGER is set to less, but lesspipe isn't configured
via LESSOPEN, then `dchange $package` fails with less like this:

% dchange zsh
"/usr/share/doc/zsh/changelog.Debian.gz" may be a binary file.  See it anyway?

vi(m) handles *.gz just fine, so let's try to use vi if PAGER is set to
less and LESSOPEN is unset.

While at it fix usage of ${PAGER} vs ${=PAGER} to also handle
a setting like PAGER='less -Mr' and PAGER=(less -Mr), otherwise
failing with `command not found: less -Mr`.

Thanks: András Korn for review and improvement suggestions