Overhaul Makefile and header files

Make .o files in separate subdirectories for servald and library
Factor struct __sourceloc and __WHENCE__ from "log.h" into "whence.h"
Factor sid_t etc from "serval.h" into "serval_types.h"
Factor rhizome_bid_t etc from "rhizome.h" into "rhizome_types.h"
Do not include "serval.h" in library sources
Add log_stderr.c and logMessage.c for stand-alone executables