FreeBSD does not need extern char *sys_errlist[]

FreeBSD does not need extern char *sys_errlist[]

Add preprocessor directives not to try
to redefine sys_errlist[] or sys_nerr

There are already definitions:

 extern __const char *__const sys_errlist[];
 extern __const int sys_nerr;

in <stdio.h>

Actually we should have something like
NeedSysErrlist in imake definitions
to get rid of those #ifdefs.