selftests/bpf: add CSV output mode for veristat

selftests/bpf: add CSV output mode for veristat

Teach veristat to output results as CSV table for easier programmatic
processing. Change what was --output/-o argument to now be --emit/-e.
And then use --output-format/-o <fmt> to specify output format.
Currently "table" and "csv" is supported, table being default.

For CSV output mode veristat is using spec identifiers as column names.
E.g., instead of "Total states" veristat uses "total_states" as a CSV
header name.

Internally veristat recognizes three formats, one of them
(RESFMT_TABLE_CALCLEN) is a special format instructing veristat to
calculate column widths for table output. This felt a bit cleaner and
more uniform than either creating separate functions just for this.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220921164254.3630690-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>