Fix zfs-mount-generator for datasets with spaces

Alternative implementation of @rlaager's original modification
of zfs-mount-generator fix, with @chrisrd's comments. Set
IFS to be only the tab character, matching our `-H` call in
`zfs list`, allowing spaces to appear in dataset names (and
mountpoints).

Also adds comments explaining our rationale.

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #8708 
Closes #8718

etc/systemd/system-generators/zfs-mount-generator.in

@@ -22,7 +22,7 @@
 # OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 # WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
-set -ef
+set -e
 
 FSLIST="@sysconfdir@/zfs/zfs-list.cache"
 
@@ -55,7 +55,12 @@ mkdir -p "${req_dir}"
 
 process_line() {
 
-  # -o name
+  # zfs list -H -o name,...
+  # fields are tab separated
+  IFS="$(printf '\t')"
+  # protect against special characters in, e.g., mountpoints
+  set -f
+  set -- $1
   dataset="${1}"
   p_mountpoint="${2}"
   p_canmount="${3}"
@@ -175,7 +180,7 @@ process_line() {
 # Automatically generated by zfs-mount-generator
 
 [Unit]
-SourcePath=${FSLIST}/${cachefile}
+SourcePath=${cachefile}
 Documentation=man:zfs-mount-generator(8)
 Before=local-fs.target zfs-mount.service
 After=zfs-import.target
@@ -193,8 +198,8 @@ EOF
 }
 
 # Feed each line into process_line
-for cachefile in $(ls "${FSLIST}") ; do
+for cachefile in "${FSLIST}/"* ; do
   while read -r fs ; do
-    process_line $fs
-  done < "${FSLIST}/${cachefile}"
+    process_line "${fs}"
+  done < "${cachefile}"
 done