Fix remounting /system on Android.

Fix remounting /system on Android.

This adds the missing "relatime" option, fixes error handling logic (mount
returns 0/-1, not an errno value, and I at least get EACCES rather than
EROFS in the case where the underlying block device is read-only; we should
also probably only try again if the ioctl actually succeeded), and adds the
missing newline in case where we go around and try again read-only.

The test case was "mount -o rw,remount /system" on Android.