Improve description of an address that is on a stack but below sp.

Improve description of an address that is on a stack but below sp.

An address below the sp will be described as being on a stack, but below sp.

The stack for such an address is found in the registered stacks.

Also, if there is a guard page at the end of the stack (lowest address)
an address in this page will be described as being in thread guard page.
A guard page is recognised as being a page not readable/writable/executable.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14399