SvxRelativeField: set the unit from the passed stringmap

In the Paragraph "Indents & Spacing" tab there are fields with unit:
<object class="svxlo-SvxRelativeField" id="spinED_LEFTINDENT:0cm">

But the unit is effectively ignored, and the resulting SvxRelativeField
with FUNIT_NONE and 9999 max somehow results in 0.18cm max when the
dialog sets cm as unit.

(cherry picked from commit 68bc8af6b6c4d0399bd10ed450d81aa2f17931a1)

Conflicts:
	svx/source/dialog/relfld.cxx

Change-Id: I6001c93079fd7d899f1e119913b29ea4898b1c34
Reviewed-on: https://gerrit.libreoffice.org/9416
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>