use PR_SET_MM_MAP instead of PR_SET_MM

use PR_SET_MM_MAP instead of PR_SET_MM

PR_SET_MM_MAP can be called as non-root, which we are in the unprivileged
(or nested) case.

Also, let's not do the strcpy() for the new cmdline until after we're sure
the prctl succeeded. This means that even if it does fail, we won't
mutilate the command line like we did before, it just won't be as pretty.

v2: remember to chop off bits of the string that are too long

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>