xlink: replace awkward fatalerror() with standard errx(3)

fatalerror() prints a given string to stdout and exits(5). It
cannot format text, so there was a string temptext used with
sprintf() to format text and pass it to fatalerror().

errx() can format text, and it prints to stderr.

As a bonus, this fixes the linking warnings from use of sprintf().

include/link/main.h

@@ -4,8 +4,6 @@
 #include "link/types.h"
 
 extern void PrintUsage(void);
-extern void fatalerror(char *s);
-extern char temptext[1024];
 extern SLONG fillchar;
 extern char smartlinkstartsymbol[256];