bpf: add sendmsg option for testing BPF programs

When testing BPF programs using sockmap I often want to have more
control over how sendmsg is exercised. This becomes even more useful
as new sockmap program types are added.

This adds a test type option to select type of test to run. Currently,
only "ping" and "sendmsg" are supported, but more can be added as
needed.

The new help argument gives the following,

 Usage: ./sockmap --cgroup <cgroup_path>
 options:
 --help         -h
 --cgroup       -c
 --rate         -r
 --verbose      -v
 --iov_count    -i
 --length       -l
 --test         -t

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>