zdtm: extent zdtm to execute tests in userns

time bash test/zdtm.sh  -i 0 -x cwd -x maps -x sched -x sock -x fanot ns/.*
restore: use new interface to set mm arguments
set PR_SET_DUMPABLE to have access to proc files
userns: save uid-s from a target userns

v2: don't leak maps
    convert uids-s after may_dump()
sysctl: hack