pmc: Initialise and check the pm_flags field for CONFIGURELOG

[jrtc27]

Summary

Whilst the former is not breaking, the latter is, and so this will be
followed by a bump to the pmc major version. This will allow the flags
to actually be usable in future, as otherwise we cannot distinguish
uninitialised stack junk from a deliberately-initialised value.

[mhorne]

It is annoying, but this represents a new distinct error return from the syscall, and should be added to the ERRORS list in hwpmc(4).

[jrtc27]

Add error case to hwpmc(4)