lldpcli: root is privileged

For some reason, this bug was not catched before. It was expected that
`lldpcli` can be installed as suid `_lldpd` to give user read access to
the neighbor information. However, to avoid write access, we checked if
the user was privileged by checking equality of effective and real user
uid. However, even for root, they don't have to match. Maybe they
matched in the past?

Therefore, directly check for UID == 0 too.

This should fix #56
lldpcli: check for privileges by checking access to control socket

Instead of relying on having used the suid bit to run the program, we
check if the real UID/GID could have opened the control socket. This
really fix #56.
lldpcli: fix readline() wrapper to take prompt as argument

`readline()` is expecting the prompt as argument. We fix the wrapper to
have the same requirement.