Reintroduce priority for the TRIM ZIOs instead of using the "NOW" priority

The changes how TRIM requests are generated to use ZIO_TYPE_FREE + a priority
instead of ZIO_TYPE_IOCTL, until processed by vdev_geom; only then is it
translated the required geom values. This reduces the amount of changes
required for FREE requests to be supported by the new IO scheduler. This
also eliminates the need for a specific DKIOCTRIM.

Also fixed FREE vdev child IO's from running ZIO_STAGE_VDEV_IO_DONE as part
of their schedule.

As the new IO scheduler can result in a request to execute one type of IO to
actually run a different type of IO it requires that zio_trim requests are
processed without holding the trim map lock (tm->tm_lock), as the free request
execute call may result in write request running hence triggering a
trim_map_write_start call, which takes the trim map lock and hence would result
in recused on no-recursive sx lock.

This is based off avg's original work, so credit to him.

MFC after:	1 month
Allow FDT_DTS_FILE to be a list, either in the makedtb target, or in a
kernel config file. If you also want to have a static DTB compiled
into your kernel, however, it cannot be a list. We have no mechanism
in the kernel for picking one, so that doesn't make sense and will
result in a compile-time error.
Omit from the universe build all config files tagged with
#NO_UNIVERSE. Many of these config files are important examples, but
add little to no regresive value to the intended purpose of
UNIVERSE. We now build over 120 kernels during universe. There's
really little to no value to this over building say 60 or even 30 of
them (either is still a way too big number). This is especially true
for kernels that are nothing more than including a common base and
adding a static DTB file. Start by pruning 1/3 of the arm kernels that
add little regresion value.
This was copied to IMX6, which has since evolved further. Remove this
as it is no longer needed.
kldxref: Clean up error reporting

Omit "too many sections" warnings if the ELF file is not dynamically
linked (and is therefore skipped anyway), and otherwise output it only
once.  An errant core file would previously cause kldxref to output a
number of warnings.

Also introduce a MAXSEGS #define and replace literal 2 with it, to make
comparisons clear.

Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation