In order to become able to generate syntax tree nodes on the roff(7)

In order to become able to generate syntax tree nodes on the roff(7)
level, validation must be separated from parsing and rewinding.
This first big step moves calling of the mdoc(7) post_*() functions
out of the parser loop into their own mdoc_validate() pass, while
using a new mdoc_state() module to make syntax tree state handling
available to both the parser loop and the validation pass.