checkpatch: add new warnings to author signoff checks.

The author signed-off-by checks are currently very vague.  Cases like same
name or same address are not handled separately.

For example, running checkpatch on commit be6577af

 ("parisc: Add
atomic64_set_release() define to avoid CPU soft lockups"), gives:

WARNING: Missing Signed-off-by: line by nominal patch author
'John David Anglin <dave.anglin@bell.net>'

The signoff line was:
"Signed-off-by: Dave Anglin <dave.anglin@bell.net&gt;">

Clearly the author has signed off but with a slightly different version
of his name. A more appropriate warning would have been to point out
at the name mismatch instead.

Previously, the values assumed by $authorsignoff were either 0 or 1
to indicate whether a proper sign off by author is present.
Extended the checks to handle four new cases.

$authorsignoff values now denote the following:

0: Missing sign off by patch author.

1: Sign off present and identical.

2: Addresses and names match, but comments differ.
   ...