loader.conf: Fix ACPI DSDT overriding.

Also, fix the instructions in the acpi(4) manual page.
acpi.4: Refresh COMPATIBILITY and update the spec reference.
acpi.4: Remove COMPATIBILITY altogether.

It can be brought back once we support more platforms.
kernel/acpi: Remove unneeded kprintf when overriding DSDT.

ACPI will print a notice ("Logical table override...") itself.
kernel/acpi: Add missing header to the Makefile.
kernel/acpi: Use ACPI_COMPARE_NAME().