pmie: do not default to '?' on a failed hostname lookup
pmie: do not default to '?' on a failed hostname lookup

Observed on Marko's systems, a transient failure to resolve
host names can result in a long-running pmie process with a
hostname set to '?' (from an error-but-continue path).  Its
far more sensible to exit here, and let the restart-regime-
du-jour fire up a new pmie when circumstances have changed,
rather than live on (potentially for many weeks/months) in
this invalid state.

andyvand/cygpcpfans,andyvand/cygpcpfans,adfernandes/pcp,mbaldessari/pcp,edwardt/pcp,mbaldessari/pcp,adfernandes/pcp,andyvand/cygpcpfans,prasincs/pcp,mbaldessari/pcp,prasincs/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,tjanez/pcp,andyvand/cygpcpfans,prasincs/pcp,wuliming/pcp,edwardt/pcp,edwardt/pcp,edwardt/pcp,andyvand/cygpcpfans,wuliming/pcp,adfernandes/pcp,edwardt/pcp,prasincs/pcp,adfernandes/pcp,tjanez/pcp,mbaldessari/pcp,tjanez/pcp,wuliming/pcp,adfernandes/pcp,aeg-aeg/pcpfans,aeg-aeg/pcpfans,wuliming/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,tjanez/pcp,prasincs/pcp,tjanez/pcp,adfernandes/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,wuliming/pcp,adfernandes/pcp,wuliming/pcp,prasincs/pcp,aeg-aeg/pcpfans,mbaldessari/pcp,tjanez/pcp,prasincs/pcp,edwardt/pcp,tjanez/pcp,edwardt/pcp,edwardt/pcp,aeg-aeg/pcpfans,wuliming/pcp,mbaldessari/pcp,adfernandes/pcp,andyvand/cygpcpfans,prasincs/pcp,wuliming/pcp,tjanez/pcp