priv: drop most privileges in monitor, only keep CAP_NET_RAW/ADMIN

priv: drop most privileges in monitor, only keep CAP_NET_RAW/ADMIN

On Linux, we mostly rely on CAP_NET_RAW. Only keep that one. However,
we also write to ifalias, which needs CAP_NET_ADMIN. We could let user
choose at runtime if they want to grant this capability or not.
Currently, a user can turn it on/off at any time.

Access to SNMP socket may also be problematic. We need some solid
solution about that before merging.

Is it safe to use the same UID for the monitored and the unprivileged
process? Signals are mostly harmless. As for ptrace, since the
monitored process as more capabilities, this will not be allowed by
Linux.