[patch] permit including other files in bird.conf

Hello list! bird filters/functions are very powerful mechanism for managing route announces and redistribution. However, building complicated filters or/and applying filters on peers based on RIPE (or MERIT, or ..) database makes bird.conf grow very fast. Additionally, all this machine-generated stuff needs to be in the beginning since config parser requires filters to be defined _before_ using them. As a result, human re-configuration becomes more complicated. Proposed solution is to add include directive to permit (recursive) include of other files from bird[6].conf. Patch is attached, comments/reviews are welcome. http link: http://static.ipfw.ru/patches/bird_includes_20110513.diff -- WBR, Alexander MELI-RIPE