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/cmd/ksh93/edit/emacs.c

@@ -1007,7 +1007,7 @@ static int escape(register Emacs_t* ep,register genchar *out,int count)
 			else if(i=='=' || (i=='\\' && out[cur-1]=='/'))
 			{
 				draw(ep,REFRESH);
-				if(count>0)
+				if(count>0 || i=='\\')
 					ep->ed->e_tabcount=0;
 				else
 				{