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

new regression tests for mdoc_macro.c revs. 1.167-1.172

one less groff-mandoc difference after mdoc_validate.c rev. 1.234

Bring .Pp/.Lp handling inside .Nm blocks closer to groff;
as a bonus, get rid of another call to rew_sub().

When a full block macro gets closed out by a mismatching
block closure macro it calls, do not attempt to open its body.
This can for example happen for (nonsensical) constructions like
.Fo
.Nm Fc
in the SYNOPSIS. Fixing an assertion failure jsg@ found with afl
some time ago (test case number 731).

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@.

Segfault fixes from kristaps@.
Note that .It and .Nm blocks without bodies cannot be generated by
valid mdoc(7) syntax but are a adequate representations of invalid
mdoc(7) constructs like Bl -hang It Bo Sh and Sh SYNOPSIS Nm Bo Sh.

start catching up with recent code changes, but no new files yet

regression tests for prologue warnings

more tests for excessively long text in TERMP_NOBREAK mode

regression tests related to mdoc_validate rev. 1.105

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

after several improvements to -Tman went in,
enable lots of related regression tests

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

add a few simple tests found in my tree