Fix the dns availability check

It makes more sense to check for UDP port 53 than for TCP port 53,
since DNS is a UDP based protocol..
Remove ping test (bnc#901744)

Some environments do not allow to send pings. The real point of the
code was to check if DNS resolution is working. So just do a DNS
test.
Merge pull request #5 from dirkmueller/dns

Fix DNS sanity checks