fixup credits

A recent commit added to the list tcpdump contributors instead of
libpcap contributors. Put the list right and fix some sorting and
formatting while at it. The new contributors to libpcap are:

* Akos Vandra
* Andres Perera
* Ani Sinha
* Brent Cook
* Christian Svensson
* Darren Lim
* David Clark
* David Ward
* Edward Sheldrake
* George Neville-Neil
* Gustavo Zacarias
* Jiri Slaby
* Jorge Boncompte [DTI2]
* Mansour Behabadi
* Michal Sekletar
* Niko Delarich
update .gitignore