Co-authored-by: cage <cage@invalid>

It is apparently a Windows only invention and has no business being defined in
the POSIX subsystem, let alone in basic-unix.lisp.

Closes #58

Add get-stream-fd on CCL and ECL

Requires a working GET-STREAM-FD, which we only have on those implementations.

Now that most things are implemented on windows as well, there's no need to
keep separate files for the high-level interface.

On non-Windows platforms there is no need to change the current directory; we
can just merge with *D-P-D* and absolutize.

Additionally, specify that the target is merged with the link's
directory. Otherwise, it's possible to inherit the name or type from link if it
is unspecified in target, which is probably not what anyone wants.

It's present, just not always useful

Addittionally, mark all functions that depend on that as supported

Involves some slight modifications of tests and completely disabling some
tests.

First, look at TMP instead of TMPDIR on Windows.

Second, it's not possible to unlink a file if someone has it open, unless you
use CreateFileA/W and set FILE_SHARE_DELETE. Therefore, note that
OPEN-TEMPORARY-FILE may return a pathname as a second value. If it does, the
user is reponsible to delete that file.

Third, modify WITH-TEMPORARY-FILE to unlink the file.

It signaled a non-existent condition type

There is no setenv, nor unsetenv, but we can emulate both of them using putenv.

Some functionality of osicat.lisp cannot be compiled on Windows without a
WARNING being signaled, due to missing POSIX functions. Move everything that
works on Windows into basic-osicat.lisp, keep everything else in
osicat.lisp (which is not loaded on Windows) and add a windows.lisp file that
marks functions as unsupported.

Additionally, MAKEDEV, MAJOR, and MINOR come from sys/sysmacros.h on Linux, but
sys/types.h on BSD.