checkpatch: add a --strict check for utf-8 in commit logs

Some find using utf-8 in commit logs inappropriate.

Some patch commit logs contain unintended utf-8 characters when doing
things like copy/pasting compilation output.

Look for the start of any commit log by skipping initial lines that look
like email headers and "From: " lines.

Stop looking for utf-8 at the first signature line.

Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>