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.

enable -T markdown tests of filled displays and tagged lists

first batch of -T markdown tests

The .Sm macro accepts at most one argument;
fixing an assertion failure found by jsg@ with afl.

Cleanup with respect to bad macro arguments.
* Fix .Sm with invalid arg: move arg out and toggle mode.
* Promote "unknown standard" from WARNING to ERROR, it loses information.
* Delete MANDOCERR_BADWIDTH, it would only indicate a mandoc(1) bug.
* Do not report MANDOCERR_BL_LATETYPE when there is no type at all.
* Mention macro names, arguments and fallbacks.

When .Sm is called without an argument, groff toggles the spacing mode,
so let us do the same for compatibility. Using this feature is of
course not recommended except in manual page obfuscation contests.

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

more tests found in my other tree

Check that end of .It does not restore .Sm state
to the value it had before entering .It;
fixed by kristaps@ in release 1.9.15.