Fix confusion between RERR_NOSUPPORT and RERR_UNSUPPORTED for exit codes

that indicate a feature is not supported.  Two places that are normally
ifdefed out used RERR_UNSUPPORTED whereas one other place and errcode.h
used RERR_NOSUPPORT.  Changed them all to consistently use RERR_UNSUPPORTED.
The two things that had the bad values were #ifndef SUPPORT_LINKS and
#ifdef NO_INT64.  The former is probably for non-Unix operating systems
and the latter was at least on the default Unixware compiler.