pager: do not read from environment variable $PAGER is converted to the pager.pager config item. So it's no longer necessary to read $PAGER in ui.pager().

ui: simplify geteditor Now $EDITOR and $VISUAL will affect ui.editor directly. So it's no longer necessary to test them in ui.geteditor.

debugconfig: list environment variables in debug output Since we print "read config from" for config files, printing environment variables will make it more consistent.