x86: Prefer consistent naming for loader tunables

[zlei]

Summary

The following loader tunables do have corresponding sysctl MIBs but
with inconsistent naming. That may be historical reason. Let's prefer
consistent naming for them so that the code is easier to read and maintain.

1. hw.dmar.timeout -> hw.iommu.dmar.timeout
2. hw.lapic_eoi_suppression -> hw.apic.eoi_suppression
3. hw.lapic_tsc_deadline -> hw.apic.timer_tsc_deadline
4. hw.x2apic_enable -> hw.apic.x2apic_mode

Those tunables are for field debugging, no need to keep old names for
compatibility.

MFC after: 5 days

[zlei]

sys/x86/x86/local_apic.c

213

@kib Are these for debugging only ?

If yes I think there's no need to flag them with CTLFLAG_TUN.

496

If yes I think there's no need to flag them with CTLFLAG_TUN.

Then no need for backward compatibility.

[kib]

Do not provide backward compat for these knobs. They are for field debugging.

[zlei]

Removed CTLFLAG_TUN, dropped code for compatibility.