fixup credits

A recent commit replaced tcpdump CREDITS with libpcap CREDITS and added
to the replaced list new libpcap contributors. Restore the original list
and add new tcpdump contributors:

* Bram
* Dmitrij Tejblum
* fra
* Gregory Detal
* Jorge Boncompte [DTI2]
* Longinus00
* Marc Abramowitz
* Ola Martin Lykkja
* Oleksij Rempel
* Petar Alilovic
* Stephane Bortzmeyer
* Thomas Jacob
* Udayakumar
* Wim Torfs

Fix some sorting and formatting while at it.