improved perf_event_open() sysfs parser

The following updates the perf_event_open() sysfs parser to fully
implement the ABI as described in the kernel ABI documentation.

This includes non-contiguous format field values, as well as support
for the config2 attribute.

This should fix the warning about commas in the format field.

Signed-off-by: Vince Weaver <vincent.weaver@maine.edu>