tests/fstests: Remove duplicate begin/end messages

Fix file system names.  Remove superfluous defines.
tests/libtests: Use <rtems/test.h>
cpukit/shell: Replace task variables with posix keys.

Use posix keys for current shell environment instead of task variables. With
this patch the shell needs one posix-key and one posix-key-value-pair
configured.

Update documentation for the shell.

Adapt samples/fileio:
- Add necessary objects.
- Add login function and custom device name for better testing of the shell.