Per rmind@'s suggestion, avoid an acquire/release-mutex dance by
collecting garbage in two phases:  in the first stage, with
alldevs_mtx held, gather all of the objects to be freed onto a
list.  Drop alldevs_mtx, and in the second stage, free all the
collected objects.

Also per rmind@'s suggestion, remove KASSERT(!mutex_owned(&alldevs_mtx))
throughout, it is not useful.

Find a free unit number and allocate it for a new device_t atomically.
Before, two threads would sometimes find the same free unit number
and race to allocate it.  The loser panicked.  Now there is no
race.

In support of the changes above, extract some new subroutines that
are private to this module: config_unit_nextfree(), config_unit_alloc(),
config_devfree(), config_dump_garbage().

Delete all of the #ifdef __BROKEN_CONFIG_UNIT_USAGE code.  Only
the sun3 port still depends on __BROKEN_CONFIG_UNIT_USAGE, it's
not hard for the port to do without, and port-sun3@ had fair warning
that it was going away (>1 week, or a few years' warning, depending
how far back you look!).

(dyoung)