where we had not hit global dirty limits but a single queue was starved
for space by dirty buffers.  A single buf_daemon is maintained for now.

Add a bd_speedup() when we are low on bufspace.  This can happen due to SUJ
keeping many bufs locked until a cg block is written.  Document this with
a comment.

Fix sysctls to work with per-domain variables.  Add more ddb debugging.

Reported by:	pho
Reviewed by:	kib
Tested by:	pho
Sponsored by:	Netflix, Dell/EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D14705