better error reporting for .br .fi .nf with arguments

Clean up warnings related to macros and nesting.
* Hierarchical naming of enum mandocerr items.
* Improve the wording to make it comprehensible.
* Mention the offending macro.
* Garbage collect one chunk of ancient, long unreachable code.

In .nf mode, use the MAN_LINE flag to detect input line breaks
instead of the man_node line member. This is required to preserve
line breaks contained in user-defined macros called in .nf mode.
Found in a code audit triggered by fixing a similar issue in .TP.

test vertical spacing around filling macros;
forgot to commit on June 2 after implementing .EX

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

Refrain from throwing fatal errors for
* .br .sp .nf .fi .na with arguments - just skip the arguments
* .TH lacking arguments - use empty strings instead like groff
* .TH with excessive arguments - skip those
Reminded by Joerg Sonnenberger, ok kristaps@.

Multiple man(7) .IP and .TP fixes started during p2k10:

Affecting both -Tascii and -Thtml:
* The .IP HEAD uses the second argument as the width, not the last one.
* Only print the first .IP HEAD argument, not all but the last.

Affecting only -Tascii:
* The .IP and .TP HEADs must be printed without literal mode,
but literal mode must be restored afterwards.
* After the .IP and .TP bodies, we only want term_newln(), not
term_flushln(), or we would get two blank lines in literal mode.
* The .TP HEAD does not use TWOSPACE, just like .IP doesn't either.
* In literal mode, clear NOLPAD after each line, or subsequent lines
would get no indentation whatsoever.

Affecting only -Thtml:
* Only print next-line .TP children, instead of all but the first.

OK kristaps@ on the -Tascii part; and:
"Can you work this into man_html.c, too?"