selftests/bpf: consolidate and improve file/prog filtering in veristat

selftests/bpf: consolidate and improve file/prog filtering in veristat

Slightly change rules of specifying file/prog glob filters. In practice
it's quite often inconvenient to do `*/<prog-glob>` if that program glob
is unique enough and won't accidentally match any file names.

This patch changes the rules so that `-f <glob>` will apply specified
glob to both file and program names. User still has all the control by
doing '*/<prog-only-glob>' or '<file-only-glob/*'. We also now allow
'/<prog-glob>' and '<file-glob/' (all matching wildcard is assumed if
missing).

Also, internally unify file-only and file+prog checks
(should_process_file and should_process_prog are now
should_process_file_prog that can handle prog name as optional). This
makes maintaining and extending this code easier.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221103055304.2904589-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>