TNetXNGFile: Make it possible to disable querying for readv limits

(cherry picked from commit e71da30898eba2fbe0bab580d33332f5976343e4)
TNetXNGFile: Workaround problem with dCache returning nonsense readv parameters

(cherry picked from commit efe4de5aa3873198822112710baaf60eb0db55cf)
TNetXNGFile: Make sure to handle echoed responses to readv_ior_max and readv_iov_max

(cherry picked from commit 999a0cffd866936fbbfe740314208ad21f13d066)
TNetXNGFile: when readv_iov_max is wrong assume that readv_ior_max is wrong too

(cherry picked from commit 85078fd6f0afe9f440c80a9897bd1922e049a203)