lib/pager: Apply pager-specific fixes only when needed

lib/pager: Apply pager-specific fixes only when needed

Currently defined output filter quirk fixes and environment variable
tweaks apply to less(1) only, so let's don't apply them when the pager
is actually configured to something else.

While there, rename the less(1)-specific callback function to make
clear what it applies to, and to make adding any posible additional
pager-specific callback functions a bit easier.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>