Add configure tests and defines for getservbyname_r, getservbyport_r, andgetprotobyname_r. Use these if available, otherwise fallback to thethread-unsafe variants.Add a unit test to exercise getprotobyname, which is currently untested.TODO: - Are there any platforms which define the unsafe variants but not the   re-entrant ones? If not we can simplify the #ifdef hell somewhat. - Do the re-entrant functions have the same signature on all platforms? - These changes follow the existing code's practice: allocate a fixed-size   (and overly large) buffer, and don't properly handle the error case if it is   too small. Should this be fixed? If so should existing code also be fixed?