[SETUPLIB] Diverse fixes (incl. initialization fixes).

[SETUPLIB] Diverse fixes (incl. initialization fixes).

- Compute the installation source paths based on the full path of the
  installer program that uses the setup library.
- Add INF_STYLE_OLDNT define in infsupp.h.
- Add some (silenced) diagnostic DPRINTs.

svn path=/branches/setup_improvements/; revision=75667

- Use correct inf style flags in SetupOpenInfFileEx() calls when opening
  txtsetup.sif and unattend.inf. Technically txtsetup.sif would be
  INF_STYLE_WIN4, but since we use "$ReactOS$" as its version signature,
  it would not work when opening it with setupapi.dll functions.
  Hence this flag is combined with INF_STYLE_OLDNT too.

- Don't fail if opening the \SystemRoot symbolic link doesn't work
  (usually due to incorrect access rights); in that case, just use the
  installer image file path as the installation source path.

svn path=/branches/setup_improvements/; revision=75676