tools: bpftool: add a command to display bpftool version

This command can be used to print the version of the tool, which is in
fact the version from Linux taken from usr/include/linux/version.h.

Example usage:

    $ bpftool version
    bpftool v4.14.0

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>