Use a dedicated struct mbr to hold the GPT protective MBR,

Use a dedicated struct mbr to hold the GPT protective MBR,
either the one read from disk or the default one created
when initializing GPT.

Simplifies logic, eliminates MBR_init_GPT() and demotes
MBR_protective_mbr() to a helper function called from
GPT_read().

Nuke an unused variable in passing.

No intentional functional change.