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.
