zdtm: set the dumpable flag after setuid and setgid (v2)

setuid and setgid drops the dumpable flag, so it should be set back.
Otherwise proccesses will not able to read a few files in proc (e.g.
/proc/pid/pagemap).

This patch affects cow01:
11:50:04.429:     4: ERR: cow01.c:99: Unable to read data (errno = 13 (No such file or directory))

v2: typo fix
mm: don't add a guard page if a prev vma grows-down too

The kernel logic is described in v2.6.36-rc1-161-g7798330:
"If we've split the stack vma, only the lowest one has the guard page."

https://bugzilla.openvz.org/show_bug.cgi?id=2715
mm: map grow-down VMA-s with guard pages

In /proc/pid/maps grow-down VMA-s are shown without guard pages, but
sometime these "guard" pages can contain usefull data. For example if
a real guard page has been remmaped by another VMA. Let's call such
pages as fake guard pages.

So when a grow-down VMA is mmaped on restore, it should be mapped with
one more guard page to restore content of the fake guard page.

https://bugzilla.openvz.org/show_bug.cgi?id=2715
zdtm: check that two continuous grow-down VMA-s are restored correctly

It's a special case, because if we've split the stack vma,
only the lowest one has the guard page.
zdtm: add grow_down test cases for regular execution
zdtm: check grow-down vma-s in cow01