Move config-dependent code (recvwithttl) from net.c to mdp_net.c (new)
Move log-implementation-independent code from log.c to log_util.c (new)
Build config_test binary in Makefile
Make all #include <netinet/in.h> conditional upon HAVE_NETINET_IN_H
Remove unnecessary #include <netinet/in.h> from source files
Reformat include block in "serval.h" for readability
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.
Introduce __WHENCE__ macro and a block comment in log.h explaining it.
In "primitive" kinds of functions, rename 'whence' arguments to '__whence' and
use WHYF(), WARNF(), DEBUGF() macros instead of calling logMessage() directly.
Replaces (const char *file, unsigned int line, const char *function) arguments
to all logging functions, simplifies malloc/free tracking code in
overlay_buffer.c and Rhizome manifest alloc/free tracking in rhizome_bundle.c.
Use __HERE__ macro instead of (__FILE__, __LINE__, __FUNCTION__) everywhere.
Special __NOWHERE__ macro is equivalent to (NULL, 0, NULL).
Declare net.c functions in new "net.h" header, so log.c doesn't have to pull
in the entire "serval.h" just to use write_str().
Facilitates progress on issue #2.
