record 0x0867 is not only SheetProtection, generalize to FeatHdr()

Change-Id: I9fe0558c013641a902ce5a95c02d37be9729fe38
start reading 0x0868 FEAT enhanced protection feature

Change-Id: Id38a7629ea5ed4bbb1a7d696926335ce0bdec6a6
MS-XLS documentation uses big-endian bit diagrams

... so if it says leftmost bit 0 then test for high bit.

Change-Id: I260ff09fff66918da887742b5d177715e59548c2