gptzfsboot grew a lot after skein support was added; need knob to control bloat

[Ngie Cooper]

> What created a partition that small?

Me.

gpart up until last summer said that users should create 44kB
freebsd-boot partitions -- des@ corrected that in r303289:

-This example uses 88 blocks (44 kB) so the next partition will be
-aligned on a 64 kB boundary without the need to specify an explicit
-offset or alignment.
-The boot partition itself is aligned on a 4 kB boundary.
+We create a 472-block (236 kB) boot partition at offset 40, which is
+the size of the partition table (34 blocks or 17 kB) rounded up to the
+nearest 4 kB boundary.
 .Bd -literal -offset indent
-/sbin/gpart add -b 40 -s 88 -t freebsd-boot ada0
+/sbin/gpart add -b 40 -s 472 -t freebsd-boot ada0

Thanks,
-Ngie