Unix: Change debugging options

The old behavior was that enabling debugging did many nontrivial changes
in BIRD behavior. The patch changes it that these changes are generally
independent. Compiling with --enable-debug now just enables compile-time
debug macros, but do not automatically activate debug mode (-d) nor local
mode (-l). Debug mode with output to file (-D) do not force foreground
mode (-f), therefore there is no need for backgroud option (-b), which is
removed. Also fixes a bug when the default log target in -D mode was
stderr instead of given debug file.