simpleperf: respect inherited SIGHUP disposition.

Only register a SIGHUP handler if we didn't inherit an ignored signal
disposition (e.g. from nohup).

Bug: http://b/70689692
Test: nohup simpleperf record -t 1
      killall -HUP simpleperf
Change-Id: I6e146ca6bcd18b5c9cf63cdb17fca73f187d1364
3 files changed