Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Readjust the cache initialization code to make it more obvious that the
cache is bypassed when disabled.

Add a caching option to dump. Use -C. Note that NetBSD has a caching option
called -r but it takes 512 byte blocks instead of megabytes, and I felt a
megabytes specification would be far more useful so I did not use the same
option character.

This will *greatly* improve dump performance at the cost of possibly
missing filesystem changes that occur between passes, and does a fairly
good job making up for the loss of buffered block devices. Caching is disabled
by default to retain historical behavior.

In tests, dump performance improved by about 40% when dumping / or /usr.

Beware that dump forks and the cache may wind up being larger then you
specify, but a more complex shared memory implementation would not produce
results that are all that much better so I kept it simple for now.

MFC after: 3 days