core: setns_fds is an array on stack

So it makes not sense to check if it's set to something. Instead
check just check setns_fds_count.

Reported by Coverity as CID #1142188, #1142187.
uwsgi_app.chdir is an array

So check if it's first member is not 0 instead.

Fix Coverity CID #971000, #970998.
Merge pull request #493 from xrmx/setns_array2

core: setns_fds is an array on stack
emperor: Avoid integer overflow

Reported by Coverity as CID #971237, #971238.
Merge pull request #494 from xrmx/coverity-2312-4

More Coverity fixes