bsd/os x: detect `top` version or switch to something more neutral

[barneygale]

• freebsd 9: http://deimos.eos.uoguelph.ca/sareibi/TEACHING_dr/ELE648_html_dr/outline99/docs/LABS_dr/top.txt
• os x: https://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/top.1.html