This commit touches neither boot/ocamlc nor boot/ocamllex
It has the side-effect of fixing the cleanup rules which did not use the
$(EXE) extension when removing a file although it was produced with the
$(EXE) extension.
* a new test ocamldoc
* three new modifiers: html, latex and man for each ocamldoc backend
* new variables:
* plugins
* skip_header_lines, skip_header_bytes
* ocamldoc_{backend,exit_status,flags,output,reference}
* make it possible to skip lines in the result file in text mode
* check_output reads skip_header_{lines,bytes} when comparing
output and reference files