acpidump(8): Use the hint.acpi.0.rsdp kenv to get at the RSDP.

acpidump(8): Use the hint.acpi.0.rsdp kenv to get at the RSDP.

Using the kernel env variable instead of the sysctl can be useful
in some circumstances, for example when ACPICA is deactivated
because a box won't run with it active, yet we want to dump the
ACPI tables to investigate the problem. The kernel env variable,
which is set by the loader, is set even if the ACPICA module
is not loaded or ACPICA has been deactivated completely.

Of course, this won't help if there is a problem with finding the
RSDP itself.

I'll submit this commit upstream to Intel.