libsysutil: frequent native crash /system/bin/vold

regression from commit a6e965578e44f9ae5f98de822ba5decec381dffc

* wrap writev with sigaction SIG_IGN SIGPIPE to emulate the
  send(,,,MSG_NOSIGNAL) call it had replaced.

(cherry pick from commit 83fc720785c4e40f3341daf7c0bf5ee99261fee9)

BUG: 12796279
Change-Id: I14363630ada79c0a5b85bb6b2afd0a1c4d5c3109