Fix nagging make_dev() warnings

[Frank W. Josellis]

All right, the problem and any related questions are gone. Thanks.

Just let me state two remarks which you may or may not find useful when 
cleaning up the code some day:

(1) The comment in src/sys/sys/conf.h, line 87, can make believe that 
SI_ADHOC serves to distinguish between make_dev() created devices and 
others (in a way complementary to FreeBSD's SI_NAMED). The role of 
SI_ADHOC turns factually out to be totally different from that, and one 
might think of a better description here.

(2) In src/sys/kern/kern_conf.c, line 49, cdevsw_ALLOCSTART is defined, 
but this is nowhere else used. Looks like old stuff that somebody should
have removed from FreeBSD-4 long ago.

Regards,
Frank Josellis