bpftool: Set binary name to "bpftool" in help and version output

Commands "bpftool help" or "bpftool version" use argv[0] to display the
name of the binary. While it is a convenient way to retrieve the string,
it does not always produce the most readable output. For example,
because of the way bpftool is currently packaged on Ubuntu (using a
wrapper script), the command displays the absolute path for the binary:

    $ bpftool version | head -n 1
    /usr/lib/linux-tools/5.15.0-50-generic/bpftool v5.15.60

More generally, there is no apparent reason for keeping the whole path
and exact binary name in this output. If the user wants to understand
what binary is being called, there are other ways to do so. This commit
replaces argv[0] with "bpftool", to simply reflect what the tool is
called. This is aligned on what "ip" or "tc" do, for example.

As an additional benefit, this seems to help with integration with
Meson for packaging [0].

[0] https://github.com/NixOS/nixpkgs/pull/195934



Suggested-by: Vladimír Čunát <vladimir.cunat@nic.cz>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221020100300.69328-1-quentin@isovalent.com