main: move getMasterKey into initFuseFrontend
main: move getMasterKey into initFuseFrontend

This is where the results are used, so call it
there.

This simplifies doMount a bit because we can get
rid of the block protecting masterkey.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs