runtime(sh,zsh): clear $MANPAGER in ftplugin before shelling out

runtime(sh,zsh): clear $MANPAGER in ftplugin before shelling out

Say you use Vim and set MANPAGER='vim -M +MANPAGER --not-a-term -'; then
:{Zs,S}hKeywordPrg (or K) will crap out and spew terminal garbage into
less when bash's "help" fails. This was introduced by 2f25e40b1
(runtime: configure keywordpg for some file types (#5566), 2023-08-23)
and may be present in other files touched by that commit.

Make the "man" invocation sensible by unsetting MANPAGER in the
environment.

Note that changing MANPAGER for `:terminal` is not needed; Vim within
Vim is perfectly fine.

closes: #14679

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>