Add targets 'dvi' and '%.dvi'

'dvi' target works using the same subroutine 'compile_primary'.
Now 'compile_primary' uses a SHELL variable $ext instead of a
Makefile variable $(ext). This enables us to use this subroutine
both to produce DVI and PS (or PDF for pdflatex) files.

While at it, remove extra 'pfail' variable

While at it, remove extra dependencies from %.ps and %.pdf targets

Commands 'make dvi' or 'make file.dvi' fail when used in
USE_PDFLATEX=true mode