ip: add support for seg6local End.BPF action

ip: add support for seg6local End.BPF action

This patch adds support for the End.BPF action of the seg6local
lightweight tunnel. Functions from the BPF lightweight tunnel are
re-used in this patch. Example:

$ ip -6 route add fc00::18 encap seg6local action End.BPF endpoint
obj my_bpf.o sec my_func dev eth0

$ ip -6 route show fc00::18
fc00::18  encap seg6local action End.BPF endpoint my_bpf.o:[my_func]
dev eth0 metric 1024 pref medium

v2: - re-use of print_encap_bpf_prog instead of fprintf
    - introduction of "endpoint" keyword for more consistency with
      others parameters

Signed-off-by: Mathieu Xhonneux <m.xhonneux@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>