fixup credits

The previous commit replaced tcpdump CREDITS with libpcap CREDITS and
added new contributors on base of that. Restore the previous contents
and and re-add:

* 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

Refine some sorting and format while at it.