The problem is that consumers of `libZFS_Core` that forget to call
`libzfs_core_init()` before calling any other function of the library
are having a hard time realizing their mistake. The library's internal
file descriptor is declared as global static, which is ok, but it is not
initialized explicitly; therefore, it defaults to 0, which is a valid
file descriptor. If `libzfs_core_init()`, which explicitly initializes
the correct fd, is skipped, the ioctl functions return errors that do
not have anything to do with `libZFS_Core`, where the problem is
actually located.
Even though assertions for that existed within `libZFS_Core` for debug
builds, they were never enabled because the `-DDEBUG` flag was missing
from the compiler flags.
This patch applies the following changes:
1. It adds `-DDEBUG` for debug builds of `libZFS_Core` and `libzfs`,
       to enable their assertions on debug builds.
2. It corrects an assertion within `libzfs`, where a function had
       been spelled incorrectly (`zpool_prop_unsupported()`) and nobody
       knew because the `-DDEBUG` flag was missing, and the preprocessor
       was taking that part of the code away.
3. The library's internal fd is initialized to `-1` and `VERIFY`
       assertions have been placed to check that the fd is not equal to
       `-1` before issuing any ioctl. It is important here to note, that
       the `VERIFY` assertions exist in both debug and non-debug builds.
4. In `libzfs_core_fini` we make sure to never increment the
       refcount of our fd below 0, and also reset the fd to `-1` when no
       one refers to it. The reason for this, is for the rare case that
       the consumer closes all references but then calls one of the
       library's functions without using `libzfs_core_init()` first, and
       in the mean time, a previous call to `open()` decided to reuse
       our previous fd. This scenario would have passed our assertion in