This is a simpler way to track dropped mids. Setting trunk=0 was more a
workaround that worked but added more purpose to the trunk field than
originally needed. The mdir's trunk usually still exists after all.
Using mid=-1 previously didn't work due to conflict with mid=-1 to
indicate an mdir is an mroot, but since removed mids only appear in the
opened-mdir list, and the opened-mdir list stores inlined mdirs as
mid=0, this is no longer a problem.
One downside of this change is we no longer get implicit NOENT behavior
from lfsr_rbyd_lookup when attempting to lookup a removed mid, but it
wasn't clear this behavior was going to be very useful...
Christopher Haster
b06d48364d