Created new console output method: sbbs_t::backfill():

it displays a string of text and fills the background color up to the
specified percentage fo the string length. I will be using it for displaying
poll results, but it could possibly be used for progress displays and such.
The colors used are configured via the "backfill" and "unfill" attributes in
ctrl/attr.cfg.

ctrl/attr.cfg

@@ -15,3 +15,5 @@ GH					Chat Text Input Locally
 G					Chat Text Input Remotely
 W					Mult-node Chat Text Input
 WH					External Programs (default attribute)
+WH5					Backfill (percentage full/done)
+WH					Unfill (percentage empty/incomplete)