Fix syntax error thrown at the point of creating the root pool, caused by

Fix syntax error thrown at the point of creating the root pool, caused by
an embedded newline appearing within the options string surrounded by
double-quotes. Rework the logic that goes into setting dataset options on
the root pool dataset while we're here -- added two new variables (which
can be altered via scripting) ZFSBOOT_POOL_CREATE_OPTIONS and also
ZFSBOOT_BOOT_POOL_CREATE_OPTIONS for setting pool/dataset attributes at
the time of pool creation. The former is for setting options on the root
pool (zroot) and the latter is for setting options on the optional separate
boot pool (bootpool) implicitly enabled when using either GELI or MBR. The
default value for the root pool variable (ZFSBOOT_POOL_CREATE_OPTIONS) is
"-O compress=lz4 -O atime=off" and the default value for separate boot pool
variable (ZFSBOOT_BOOT_POOL_CREATE_OPTIONS) is NULL (no additional options
for the separate boot pool dataset).

Reviewed by:	allanjude

git-svn-id: svn://svn.freebsd.org/base/stable/10@267061 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f