kern: replace several EBADF with EINVAL

[kib]

Summary

EBADF semantic is that the passed fd is invalid, not that it is of wrong
type.  Using EBADF in these places in kern_event.c and sys_procdesc.c
give bad examples to copy from.

Note that places in kern_event.c that checks KQ_CLOSING and return EBADF
are kept, since KQ_CLOSING is the transient state before the fd is
finally closed and become eligible for EBADF.