samba-tool gpo: better entities check copes with new lines

samba-tool gpo: better entities check copes with new lines

Per https://www.w3.org/TR/xml/#sec-entity-decl (and MS references)
there is always some whitespace between '<!ENTITY' and the name, and
between the name and whatever is next. Also, it is valid XML to have
newlines inside entity declarations, like this:

<!ENTITY
    bubble
      "*S-1-5-113"
      >

We used to create such files, so we should allow them.

There is a kind of entity that has '%' before the name, and there are
non-ascii names, which we continue not to support.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15829

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Jennifer Sutton <jennifersutton@catalyst.net.nz>