Start systematic improvements of error reporting.
So far, this covers all WARNINGs related to the prologue.

1) hierarchical naming of MANDOCERR_* constants
2) mention the macro name in messages where that adds clarity
3) add one missing MANDOCERR_DATE_MISSING msg
4) fix the wording of one message related to the man(7) prologue

Started on the plane back from Ottawa.
Infrastructure for regression tests of mandoc messages.
Even though messages are not going to be as stable as formatted output,
that is, even though the *.out_lint files are expected to change now and then,
from now on, i want to be able to catch unintended changes in messages.