checkpatch: warn if missing author Signed-off-by

Print a warning if none of the Signed-off-by lines cover the patch author.

Non-ASCII quoted printable encoding in From: headers and (lack of) double
quotes are handled.  Split From: headers are not fully handled: only the
first part is compared.

[geert+renesas@glider.be: only encode UTF-8 quoted printable mail headers]
  Link: http://lkml.kernel.org/r/20180718145254.4770-1-geert+renesas@glider.be
Link: http://lkml.kernel.org/r/20180712100323.26684-1-geert+renesas@glider.be


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Joe Perches <joe@perches.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>