liblog: fix fd leakage

File descriptors remain open across an exec unless FD_CLOEXEC is
set.  Add O_CLOEXEC to the open() call to prevent file descriptor
leakage.

In particular, the following program will eventually run out of
file descriptors:

int main(int argc, char **argv) {
  printf("===== entering main =====\n");
  ALOGW("entering main");
  system("ls -l /proc/self/fd/");

  execv(argv[0], argv);
  printf("exec failed\n");
  return -1;
}

Change-Id: I5be43ab3b9f82a05f242b1f586454c50568af388