dblatex: purify and add dblatexFull attribute

imagemagick, transfig, inkscape, fontconfig and ghostscript are missing
dependencies of dblatex. Instead of adding all those dependencies to the
existing dblatex attribute, make a new dblatexFull attribute for that.

Also pass --use-python-path at install time so that script shebangs end
up with #!/path/to/python instead of #!/path/to/env python (which is
impure when not run in a wrapper).