Dynamic ddb commands

[Matthew Fleming]

> In general it is far easier to just add sysinit's than to hack directly on the 
> kernel linker.  There are very few ddb commands, so one extra pointer or two 
> per command is not a lot of space.

Respectfully, I disagree, for several reasons.

First, in order to make sysinit and sysctl work, the kernel linker
needed to know that there are a set of elf sections that have special
meaning.  Yes, using sysinits means that there are still only two elf
sections of interest.

Second, as I mentioned before, having ddb commands added mixed in with
sysinits means that, if I have a bug in my sysinit I may not be able to
use some of my ddb commands to debug it.  Even if DB_*COMMAND used
SI_ORDER_FIRST, any sysinit with the same priority may come first.

Next, if you want commands sorted globally, it could be done with either
implementation.  But I think that commands that are defined by a module
should be listed with others from that module.

Last, changing struct command introduces a binary compatibility issue.
Any older driver that had a ddb command (even if they never realized
they couldn't access it) would need to be recompiled.  I am not sure of
FreeBSD's binary compatibility policy, though, and it would presumably
be across a major OS revision number.  So this is not a very compelling
argument.

Thanks,
matthew