This is a SysprofInstrument that will eventually contain the contents of
the sysprof-battery-source.c to get battery charge/rate/etc. Note that
this is different from energy usage which will come from RAPL.
This starts porting the old perf code from libsysprof into
libsysprof-profile. It does not bring over tracepoints yet, because we
need to think about how we want to support that. I may opt to do that as
a separate instrument even if that takes an additional perf fd.
The goal here is to do the whole thing on a fiber rather than how we were
doing it before. This just gets the counter registration going, but we
need to follow up with the parsing/lseek/etc code.
