pmc: Initialise and check the pm_flags field for CONFIGURELOG

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.

Reviewed by: jhb, mhorne
Differential Revision: https://reviews.freebsd.org/D40049