Fix warnings and 64-bit problems in openbsd-malloc code

The warning fixes are:
  - Only define issetugid if it's missing.
  - Explicitly ignore the return value of writev.
  - Explicitly cast the retval of readlink() to int.

The 64-bit problems are related to just storing a size_t in an int. Not cool!  Use a size_t instead.

Fix for bug 6379. Bugfix on 0.2.0.20-rc, which introduced openbsd-malloc.