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: AndrĂ¡s Korn for review and improvement suggestions