Rework makefile in something closer to upstream Makefile to ease maintainance
Rework mandoc Makefile to ease maintainance
Add compat_reallocarray into libmandoc given other mandoc components will use
it.
Add the preconv utility from mandoc project

it recodes multibyte UNIX manual files into mandoc(1)
Define LIBSQLITE3 and LIBMANDOC
Add the demandoc utility from the mandoc project

This is a modern version of the deroff utility, useful for example to do
spellchecking on manpages