logd: make liblogd just the core logd functionality

liblogd: LogBuffer classes and their support, LogReaderList and
LogReaderThread
logd: liblogd + the socket code that connects liblogd to liblog and
therefore the world.

The goal here is to test liblogd offline of the socket connections or
the device.

Also, convert libaudit.c -> libaudit.cpp and modernize a bit.

Test: build
Change-Id: If5adf5e775a251d9a703c0583be0988f48017347