Commit Briefs
Launch external programs through shell.
Passing $PAGER directly as argv[0] of a subprocess only works when $PAGER is simply the name of a binary file, such as "less". Users may want to hardcode command-line options, e.g. PAGER="less -N", so we need to do something like argv = {"sh", "-c", $PAGER}. The same applies to $SHELL and $EDITOR.