Return ENOEXEC instead of ENOENT if an ELF binary's interpreter doesn't exist.

If you try to execute a binary compiled for ld-linux.so.1 (libc5) on a machine
with only ld-linux.so.2 (libc6), your shell will claim "mybinary: No such file
or directory", even though the binary exists. The ENOENT actually applies to
the ELF intepreter, not to the file itself. This patch causes it to give a more
helpful and more expected error, "cannot execute binary file".

Signed-off-by: Geoffrey Thomas <geofft@mit.edu>
Tested-by: Anders Kaseorg <andersk@mit.edu>

--- linux-2.6/fs/binfmt_elf.c	2009-02-22 22:13:44.000000000 -0500
+++ linux-2.6/fs/binfmt_elf.c	2009-03-03 23:02:51.392456503 -0500
@@ -699,8 +699,11 @@
 
 			interpreter = open_exec(elf_interpreter);
 			retval = PTR_ERR(interpreter);
-			if (IS_ERR(interpreter))
+			if (IS_ERR(interpreter)) {
+				if (retval == -ENOENT)
+					retval = -ENOEXEC;
 				goto out_free_interp;
+			}
 
 			/*
 			 * If the binary is not readable then enforce