Rework makefile in something closer to upstream Makefile to ease maintainance


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274890 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Rework mandoc Makefile to ease maintainance
Add compat_reallocarray into libmandoc given other mandoc components will use
it.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274891 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Add the preconv utility from mandoc project

it recodes multibyte UNIX manual files into mandoc(1)


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274893 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f