style message about missing blank before trailing delimiter;
inspired by mdoclint(1), and jmc@ considers it useful

Now that markdown output is tested for almost everything, test all
input files in -T markdown output mode by default and only mark
those files with SKIP_MARKDOWN that are not to be tested.
Much easier to read, and almost minus 40 lines of Makefile code.

first batch of -T markdown tests

Right after .Fl, a middle delimiter triggers an empty scope,
just like a closing delimiter. This didn't work in groff-1.15,
but it now works in groff-1.22.

After being closed by delimiters, .Nm scopes do not reopen.

Do not suppress white space after .Fl if the next node is a text node
on the same input line; that can happen for middle delimiters.

Fixing an issue reported by jmc@.

fix -Tman font handling for:
.Ad .Ar .Cd .Cm .Dv .Em .Er .Ev .Fa .Fl .Fn .Fo .Ft
.Ic .In .Lk .Li .Ms .Mt .Nm .Pa .Sx .Sy .Tn .Va .Vt

Complete overhaul of the mandoc(1) test suite.
* Test the recently introduced -Tman output mode, too.
* Specify date and OS arguments in all tests.
* Remove the kludge of sed(1)ing away the page footer lines.
* New make(1) variables SKIP_TMAN, SKIP_GROFF.
* Drop obsolete, clumsy make(1) variable GROFF_TARGETS.
* Delete obsolete mdoc/Bl/E*.sh error reporting tests.
* Silence char/N/basic.
* New targets:
- ascii, ascii-clean - to run -Tascii tests only
- tman, tman-clean - to run the new -Tman tests only
- obj-clean - maintainer only, needed before groff-clean and groff
- groff-clean - maintainer only, affects checked-in files

Regression test for .Pa below .Bl in the FILES section;
still broken in mandoc 1.9.14, fix being sent to kristaps@.

Path names (.Pa) are usually underlined, except below .Bl -inset, -hang,
-ohang, -tag or -column in the FILES section (yuck).

Note that .Bl -bullet is subtly broken in an urelated way:
That will need fixing as well before this test can fully succeed.