pmc(8): resolve -Wshadow issues

- Prefix all structs with the struct keyword to avoid collisions between
  the types and variables with the same "name".
- Use `_` suffixed variables in initializers to distinguish input
  parameters from public members [1].

Resolve some trailing whitespace issues while here.

NOTE: this doesn't resolve the -pedantic issue reported by g++ with
`pmchdr_cpuidinfo::cpuid` about the field being a flexible array in an
otherwise empty struct.

1. I generally do this the other way around, i.e., suffix private/protected
   members with `_`, but these are public members in structs and I don't want
   to introduce a lot of churn in calling code.

Reported by:	g++14 with FreeBSD CI (powerpc64 tinderbox)
Fixes:	ce6ab51f ("pmc: enable the new pmc commands")

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