bgpd: implement route-map set as-path prepend last-as

It picks up the AS to add from the aspath, or uses the peers
AS number. Useful mostly in iBGP setups.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Reviewed-by: Paul Jakma <paul@opensourcerouting.org>
diff --git a/doc/bgpd.texi b/doc/bgpd.texi
index de70970..5004cbf 100644
--- a/doc/bgpd.texi
+++ b/doc/bgpd.texi
@@ -496,6 +496,7 @@
 @end deffn
 
 @deffn {Route Map} {set as-path prepend @var{as-path}} {}
+@deffnx {Route Map} {set as-path prepend last-as @var{num}} {}
 @end deffn
 
 @node Private AS Numbers