Added a flag to optionally skip the multithreaded check in setns().
Added a flag to optionally skip the multithreaded check in setns().

Review: https://reviews.apache.org/r/53681/

dforsyth/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,gsantovena/mesos,asamerh4/mesos,chhsia0/mesos,asamerh4/mesos,dforsyth/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,zmalik/mesos,zmalik/mesos,reneploetz/mesos,jpeach/mesos,reneploetz/mesos,reneploetz/mesos,gsantovena/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,dforsyth/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,neilconway/mesos,zmalik/mesos,reneploetz/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,kaysoky/mesos,neilconway/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos,craimbert/mesos,dforsyth/mesos,craimbert/mesos,craimbert/mesos,andschwa/mesos,kaysoky/mesos,jpeach/mesos,gsantovena/mesos,andschwa/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,reneploetz/mesos,kaysoky/mesos,reneploetz/mesos,jpeach/mesos,chhsia0/mesos,abudnik/mesos,jpeach/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,asamerh4/mesos,zmalik/mesos,gsantovena/mesos,neilconway/mesos,kaysoky/mesos,craimbert/mesos,Gilbert88/mesos,neilconway/mesos,Gilbert88/mesos,verizonlabs/mesos,Gilbert88/mesos,yuquanshan/customizedMesos,kaysoky/mesos,abudnik/mesos,neilconway/mesos,chhsia0/mesos,gsantovena/mesos,jpeach/mesos,abudnik/mesos,Gilbert88/mesos,zmalik/mesos,asamerh4/mesos,reneploetz/mesos,Gilbert88/mesos,craimbert/mesos,kaysoky/mesos,andschwa/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,asamerh4/mesos,asamerh4/mesos,chhsia0/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,chhsia0/mesos,yuquanshan/customizedMesos,dforsyth/mesos,andschwa/mesos,asamerh4/mesos,verizonlabs/mesos,neilconway/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,zmalik/mesos,chhsia0/mesos,abudnik/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,zmalik/mesos,abudnik/mesos,gsantovena/mesos,craimbert/mesos,gsantovena/mesos