bpftool: Use libbpf_bpf_attach_type_str

This change switches bpftool over to using the recently introduced
libbpf_bpf_attach_type_str function instead of maintaining its own
string representation for the bpf_attach_type enum.

Note that contrary to other enum types, the variant names that bpftool
maps bpf_attach_type to do not adhere a simple to follow rule. With
bpf_prog_type, for example, the textual representation can easily be
inferred by stripping the BPF_PROG_TYPE_ prefix and lowercasing the
remaining string. bpf_attach_type violates this rule for various
variants.
We decided to fix up this deficiency with this change, meaning that
bpftool uses the same textual representations as libbpf. Supporting
tests, completion scripts, and man pages have been adjusted accordingly.
However, we did add support for accepting (the now undocumented)
original attach type names when they are provided by users.

For the test (test_bpftool_synctypes.py), I have removed the enum
representation checks, because we no longer mirror the various enum
variant names in bpftool source code. For the man page, help text, and
completion script checks we are now using enum definitions from
uapi/linux/bpf.h as the source of truth directly.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220523230428.3077108-10-deso@posteo.net