3 one-line changes to allow regression tests to run on OS X / BSD, and compilation on FreeBSD

If "tac" doesn't exist (e.g., on non GNU/Linux systems like OS X & other BSDs), emulate with sed '1!G;h;$!d' (from http://www.catonmat.net/blog/sed-one-liners-explained-part-one/)

Since the Makefile executes tests using "sh" instead of "bash", use the more portable "> obs 2>&1" instead of "&> obs" so the test will pass when /bin/sh isn't bash (e.g., on Debian-based Linux distributions, and most non-Linux *nix systems).

sockaddr_in and sockaddr_in6 are defined in netinet/in.h, which may or may not be included by arpa/inet.h (per POSIX.1-2008). Explicitly include netinet/in.h to compilation on FreeBSD 9.0.

let groupby handle a header line (starting with "#") as do other

bedtools subcommands.

Groupby columns specified like cut (but retaining order)