ui:util:pager: cleanup pager implementation

ui:util:pager: cleanup pager implementation

Changes:

* Import libraries with an underscore prefix to avoid polluting the
  module's public namespace.
* Use a copy `env` to avoid messing with the child process'
  environment.
* Pass the tweaked environment on to the PAGER (e.g. for PATH).
* Handle PAGER='' (by not paging).
* Use shlex.split to handle cases like PAGER='less -FRSX' (thanks to
  Da_Blitz from pocketnix.org for the bug report and shlex idea).