If a mirrored page style is used with first-page on both a right page
and a left page the current design cannot work because the margins in
the SwPageDesc::aFirst cannot be right for both cases.

So split that up so we get a first-master and first-left format and
copy the headers/footers and margins as appropriate... which is really
adding epicycles to a flawed design; probably this would be better with
just a single SwFrmFmt that has different items for master/left/first
headers.

Change-Id: I0889a4ab5959b5a71172203bb64d185960a07d73