"valid users" options and friends only use ',' as field delimiter

"valid users" options and friends only use ',' as field delimiter

Netatalk 3 uses ',' and ' ' as field delimiter in the "valid users",
"invalid users", "rolist" and "rwlist" option for consistency with
other options that also allow the use of ' ' as field delimiter.
But as user and groupnames may contain spaces, we need to revert
to the Netatalk 2 behaviour where only ',' was used as delimiter.

Fixes bug #472.