* This is an amendment to 77f5471d to make implicit shortening
  conversions explicit (prevents compiler warnings about what we
  know is benign).

* Initializing the controling variable to -1 prevents that.
* This change also reduces the number of globally visible symbols
  the feature introduces.

* Casting from 'type **' to 'const type **' is not allowed by C.

* Since write, mmap and a number of other functions are not part of
  ISO C (i.e. strictly conforming programs must not use those), we need
  to ensure the build environment provides POSIX conformance.
* This mostly prevents unrelated bug reports later on.
* From POSIX 2008, CDI unconditionally only uses getline and has a
  work-around in case that is missing.