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).
asciidoc: purify and add asciidocFull attribute

The current asciidoc expression is impure; it relies on several tools to
be found in PATH at runtime. This commit adds a enableStandardFeatures
parameter that, if true, pulls in all dependencies and patches asciidoc
to contain full paths to the tools.

I've set enableStandardFeatures = false for the existing asciidoc
attribute so that the closure size stays unchanged, at 255 MiB. The new
asciidocFull attribute (with enableStandardFeatures = true) has a
closure size of 1.5 GiB.