C code bug fixes

- fix to buildin.c to prevent compiler from optimizing it out
- fix to realpath.c to handle case where 'path' is NULL

- fix to emacs.c (I think from dgk)

- fix to file.c (I think from gsf)
- fix to tail.c (I think from gsf)

- fix to code setting the malloc init hook (for vmalloc)

- changed memcpy to memmove in several places in sfio because src and dst
  might overlap

- fixes to macros like FILE_defined to fix header file issues
- fixes to expr.h to handle macro definition issue

src/lib/libast/sfio/sfread.c

@@ -92,7 +92,7 @@ size_t		n;	/* number of bytes to be read. 	*/
 		{	if(r > (ssize_t)n)
 				r = (ssize_t)n;
 			if(s != f->next)
-				memcpy(s, f->next, r);
+				memmove(s, f->next, r);
 			f->next += r;
 			s += r;
 			n -= r;