maint: adjust formatting of certain continued strings

maint: adjust formatting of certain continued strings

Add a rule to ding any source file that has a continued string
with a word in the first column of the following line.
Those tend to trigger malfunction in tools that try to map an
arbitrary line number to an enclosing function name.  Of course,
very many strings do precisely this, *when they are part of the
usage function*.  So we exempt the body of each usage function.
* src/dircolors.c (main): Separate a long, continued string
into two separately-quoted parts.
* src/od.c (decode_one_format): Likewise.
(decode_one_format, main): Move a space from end of
preceding line to the beginning of the continued line.
* src/tr.c (unquote, string2_extend, validate): Likewise.
* src/seq.c (main): Split in two and use string concatenation.
* src/stat.c (default_format): Use a mix of techniques.
* cfg.mk (sc_prohibit_continued_string_alpha_in_column_1): New rule.
Exempt three files in src: system.h, od.c, printf.c.