Redo umass_bbb_get_max_lun() and uses thereof to
Redo umass_bbb_get_max_lun() and uses thereof to

1) Not bail out of umass_attach() just because the max lun cannot be
found. Assume 0 and complete attachment.

2) Not expose sc->maxlun to possible misbehaving devices. Provide a
buffer and copy it into sc->maxlun only if the attempt to get the max
lun succeeds.

3) Shrink and simplify error handling and debug output.

Prompted by the recurrance of the problem reported in PR#5478. Code
taken for the most part from FreeBSD.

ok miod@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars