MFC r306743,r317712:

r306743 (by markj):

gmirror: Bump the syncid if broken disks are found during startup.

Consider a mirror with two components, m1 and m2. Suppose a hardware error
results in the removal of m2, with m1's genid bumped. Suppose further that
a replacement mirror component m3 is created and synchronized, after which
the system is shut down uncleanly. During a subsequent bootup, if gmirror
tastes m1 and m2 first, m2 will be removed from the mirror because it is
broken, but the mirror will be started without bumping the syncid on m1
because all elements of the mirror are accounted for. Then m3 will be
added to the already-running mirror with the same syncid as m1, so the
components will not be synchronized despite the unclean shutdown.

Handle this scenario by bumping the syncid of healthy components if any
broken mirrors are discovered during mirror startup.

r317712 (by markj):

Synchronize unclean mirrors before adding them to a running gmirror.

During gmirror startup, if component mirrors are found to be dirty as is
typical after a system crash, the mirrors are synchronized to the mirror
with highest priority. However if a gmirror starts without all of its
mirrors present, for example because of some transient delays during
tasting, the remaining mirrors must be synchronized before they may become
active.