cp: give a better diagnostic for nonexistent dest/

This patch was written by Jim Meyering and myself.
* src/copy.c (copy_reg): Turn EISDIR to ENOTDIR to improve the
quality of diagnostics for commands like "cp a nosuch/".  Reported
by Марк Коренберг and Alan Curry in the thread starting at:
http://lists.gnu.org/archive/html/bug-coreutils/2010-11/msg00178.html
* THANKS: Update.
* tests/mv/trailing-slash: Add a test.

tests/mv/trailing-slash

@@ -50,4 +50,12 @@ done
 #touch a a2
 #mv a a2/ && fail=1
 
+# Test for a cp-specific diagnostic introduced after coreutils-8.7:
+printf '%s\n' \
+  "cp: cannot create regular file \`no-such/': Not a directory" \
+> expected-err
+touch b
+cp b no-such/ 2> err && fail=1
+compare err expected-err || fail=1
+
 Exit $fail