replace add_*_randomness with enqueue_randomness()

this gets rid of the source annotation which doesn't really add
anything other than adding complexitiy. randomess is generally
good enough that the few extra bits that the source type would
add are not worth it.

ok mikeb@ deraadt@

Hyper-V hosts make 64 bytes of entropy available to guests in the form
of a OEM0 ACPI table. acpihve(4) feeds this data into the kernel
entropy pool.

This is less interesting for machines with rdrand, but there are still
pre ivy bridge machines running Hyper-V (including parts of Azure).

ok mikeb@ reyk@ deraadt@ mlarkin@