Fix lesspipe.sh detection in custom PATH

The detection of lesspipe.sh has to be done after setting PATH as it
could be installed in a non-standard bin directory. For example,
Homebrew installs lesspipe in /usr/local.
[Fix #383] Do not set $MANPATH

man dynamically searches for man pages based on $PATH.
[Fix #385] Do not set $INFOPATH
[Fix #381] Remove the path_helper workaround