patman: don't mess with signoffs

Currently patman assumes that there should be only one Signoff line
and this is obviously incorrect: we often have to work with patches
containing other people signoffs. Moreover, it's really desirable
to preserve the comments between signoffs.

So until some sophisticated signoff processing will be developed I
suggest just don't mess with signoffs at all and treat them like
plain text lines. The only drawback I've found so far is the case
where you have a patch with someones else signoff but not yours and
also have to patman tags under signoff line. In this case you will
get extra empty line between signoffs.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>