manager: use more stpcpy()
util, manager: and mempset() and use it

Just like mempcpy() is almost identical to memcpy() except the useful
return value, so is the relation of mempset() to memset().
core: single unit_kill implementation for all unit types

There are very few differences in the implementations of the kill method in the
unit types that have one. Let's unify them.

This does not yet unify unit_kill() with unit_kill_context().