Preparation for a future commit where kernel's dependency
on logging.cpp is removed completely.
Replace usage of logging\.h with util/log\.h where it
suffices, and fix wrong includes according to iwyu.
Currently, there are issues with headers in `batchpriority.cpp`:
1. `SCHED_BATCH` is not defined on all supported *BSD platforms.
2. `pthread.h` is necessary on other platforms.
This addresses both issues and fixes other includes.
With the previous move of AlertNotify out of the validation file, and
thus out of the kernel library, ScheduleBatchPriority is the last
remaining function used by the kernel library from util/system. Move it
to its own file, such that util/system can be moved out of the util
library in the following few commits.
Moving util/system out of the kernel library removes further networking
as well as shell related code from it.
