README: show how to use tcpdump for troubleshooting
packaging: update to 0.7.6
packaging: change `PreReq` to `Requires(pre)` in lldpd.spec

PreReq is deprecated and makes the build fail on OSC.
osx: fix postinstall script

`@PRIVSEP_USER@` and `@PRIVSEP_GROUP@` were not substituted with the
appropriate values because they were not declared as an output
variable.
osx: update SHA1 sum for Homebrew recipe