Add support for NetBSD.

[fraggerfox]

This patch adds support NetBSD and was tested in NetBSD 9.2.

$ ./main
http://www.google.com
/usr/pkg/bin/xdg-open: line 871: www-browser: command not found
/usr/pkg/bin/xdg-open: line 871: links2: command not found
/usr/pkg/bin/xdg-open: line 871: elinks: command not found
/usr/pkg/bin/xdg-open: line 871: links: command not found
/usr/pkg/bin/xdg-open: line 871: lynx: command not found
/usr/pkg/bin/xdg-open: line 871: w3m: command not found
xdg-open: no method available for opening 'file:///tmp/browser.1539593927.html'
2021/09/09 19:05:00 exit status 3
$ uname -a
NetBSD netbsd-build 9.2 NetBSD 9.2 (GENERIC) #0: Wed May 12 13:15:55 UTC 2021  mkrepro@mkrepro.NetBSD.org:/usr/src/sys/arch/amd64/compile/GENERIC amd64

And it opens one of them if they are present.

[davecheney]

TYVM