bpf: selftests: Add helpers to directly use the capget and capset syscall

After upgrading to the newer libcap (>= 2.60),
the libcap commit aca076443591 ("Make cap_t operations thread safe.")
added a "__u8 mutex;" to the "struct _cap_struct". It caused a few byte
shift that breaks the assumption made in the "struct libcap" definition
in test_verifier.c.

The bpf selftest usage only needs to enable and disable the effective
caps of the running task. It is easier to directly syscall the
capget and capset instead. It can also remove the libcap
library dependency.

The cap_helpers.{c,h} is added. One __u64 is used for all CAP_*
bits instead of two __u32.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20220316173823.2036955-1-kafai@fb.com