libbpf: Deprecate bpf_program__load() API

Mark bpf_program__load() as deprecated ([0]) since v0.6. Also rename few
internal program loading bpf_object helper functions to have more
consistent naming.

  [0] Closes: https://github.com/libbpf/libbpf/issues/301

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211103051449.1884903-1-andrii@kernel.org