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/include/ast_std.h

@@ -76,6 +76,25 @@ struct _sfio_s;
 #undef	FILE
 #endif
 
+#ifndef FILE
+#ifndef _SFIO_H
+struct _sfio_s;
+#endif
+#define FILE            struct _sfio_s
+#ifndef __FILE_typedef
+#define __FILE_typedef  1
+#endif
+#ifndef _FILEDEFED
+#define _FILEDEFED      1
+#endif
+#ifndef ____FILE_defined
+#define ____FILE_defined 1
+#endif
+#ifndef __FILE_defined
+#define __FILE_defined 1
+#endif
+#endif
+
 /* locale stuff */
 
 #if !_hdr_locale