main: revert recognition of "b" argument

Commit 099663ff8c117303af369a4d412dafed0c5614c2 added "b" as a
recognized argument, however, B is not a runlevel like S. (B appears
as a pseudo runlevel in openSUSE's init.d scripts only for the sake of
insserv being able to manage /etc/init.d/boot.d like the other dirs).