recovery: Gracefully terminate sideload

 * When the host aborts sideload during verification, the FUSE file is
   mmap()'d and the abort generates a SIGBUS.  Handle this with longjmp
   and return VERIFY_FAILURE.

 * During adb sideload, handle SIGTERM by setting a flag.  This allows
   the sideload thread to terminate gracefully.

 * Note SIGTERM is handled by the main adb thread, so the sideload
   thread does not get an EINTR from read().  Add a select() to ensure
   that the sideload thread can detect the error.

Change-Id: I2287ca2e7178138723c437174970434bfcb3c215