tools: bpftool: unify installation directories

Programs and documentation not managed by package manager are generally
installed under /usr/local/, instead of the user's home directory. In
particular, `man` is generally able to find manual pages under
`/usr/local/share/man`.

bpftool generally follows perf's example, and perf installs to home
directory. However bpftool requires root credentials, so it seems
sensible to follow the more common convention of installing files under
/usr/local instead. So, make /usr/local the default prefix for
installing the binary with `make install`, and the documentation with
`make doc-install`. Also, create /usr/local/sbin if it does not exist.

Note that the bash-completion file, however, is still installed under
/usr/share/bash-completion/completions, as the default setup for bash
does not attempt to load completion files under /usr/local/.

Reported-by: David Beckett <david.beckett@netronome.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>