Allows user to specify an optional ZFSBOOT_POOL_SIZE for their zroot

The default is to create a zroot that consumes the whole disk because if
used with geli(8) this makes sense.

Without geli(8), I like to keep my data pool separate from my system
pool.

This is different than ZFSBOOT_BOOT_POOL_SIZE which is named bootpool.

Reviewed by: allenjude
Pull Request: https://github.com/freebsd/freebsd-src/pull/53
Differential Revision: https://reviews.freebsd.org/D30588