BUG: fix support for DCMTK DICOM dictionaries

See issue 4013, followup on commit 24473. Dictionaries passed to DCMTK in
DCMDICPATH override embedded dictionaries, and standard dictionary needs to be
specified together with the private one, as shown in example here:

http://support.dcmtk.org/redmine/projects/dcmtk/wiki/Howto_Dictionaries

This commit installs dicom.dic and initializes the environment variable.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24483 3bd1e089-480b-0410-8dfb-8563597acbee