When the 'debug.hwpstate_verbose' tunable/sysctl knob is set, dump the
initial content of the CPPC_CAPABILITY_1 and CPPC_REQUEST registers.

If, after enabling CPPC, reading/writing some MSR fails during the attach
sequence, print a diagnostic. However, once CPPC is enabled, we cannot
go back (disabling it is impossible), so we'll attach even if fiddling
with other MSRs failed.

While here, move diagnostic printing on attach out of the callback that
is executed on (potentially) another CPU and with interrupts disabled,
putting it into the attach routine itself.

Sponsored by: The FreeBSD Foundation