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
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>