lib/linux_ns: check mkdir() return value

Fail earlier if we can't create the dirs we need.
Reported by Coverity as CID #970969.
core: log when fcntl() fails

Reported by Coverity as CID #970967, #970966.
core: log when sendmsg fails

Reported by Coverity as CID #970962, #970963.
core/cluster: log sendto() error

Reported by Coverity as CID #970958.
Merge pull request #141 from xrmx/coverity5

Check return value of library call Coverity issues