bpfcountd: add initial package

bpfcountd was created to obtain packet statistics in larger networks
without stressing the cpu resources. bpfcountd will count the amount
of packages and bytes over time (for each defined rule). The rules
are defined using the tcpdump filter syntax (bpf). The collected
data is provided on a unix socket in plaintext.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>