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.
Add str.h to list of Serval header files. Make the NaCl objects not dependent
on Serval header files, so that the entire NaCl source does not get re-built
when editing serval.h or str.h etc.
