also add test command to report memory usage (RSS on OSX, VMsize
on linux, because VMsize on OSX reports all system libraries, so
reports about 2GB even for tiny programmes.)
The new header "os.h" defines a supplemental API around existing basic
operating system services (system calls and standard library functions).
Moved some function prototypes from serval.h and net.h into os.h.
This allows non-servald executables to take advantage of this API without
having to include serval.h and bringing in all the other servald link-time
baggage.