init: clean up logging initialization

Clean up a few mistakes in logging initialization

1) Only init needs to clear stdout/stderr/stdin, so remove this from
   ueventd, watchdogd, and subcontext init
2) Only init should reboot due to FATAL errors. This was true even
   before this change due to getpid() checks, but there's no reason to
   not just use the DefaultAborter for other processes.
3) It's probably a mistake for FATAL logs in init to try to gracefully
   shutdown the system, so simply call RebootSystem() here.
4) Lastly, remove log.cpp since it's not actually shared code anymore

Test: build
Change-Id: Ic8c323393dc7ee98ed6bb9691361b51d0d915267