Rework kcov kernel config. Instead of treating kcov as both an option and a

Rework kcov kernel config. Instead of treating kcov as both an option and a
pseudo-device, get rid of the option. Enabling kcov now requires the following
line to be added to the kernel config:

  pseudo-device kcov 1

This is how pseudo devices are enabled in general. A side-effect of this change
is that dev/kcov.c will no longer be compiled by default.

Prodded by deraadt@; ok mpi@ visa@