MFC r272884:

accept(2) may and can return EAGAIN, document it.
MFC r272504: MFV r272494:

Make space_map_truncate() always do space_map_reallocate().  Without
this, setting space_map_max_blksz would cause panic for existing pool,
as dmu_objset_set_blocksize would fail if the object have multiple blocks.

Illumos issues:
   5164 space_map_max_blksz causes panic, does not work
   5165 zdb fails assertion when run on pool with recently-enabled
	spacemap_histogram feature
MFC r272507: MFV r272496:

Add tunable for number of metaslabs per vdev
(vfs.zfs.vdev.metaslabs_per_vdev).  The default remains
at 200.

Illumos issue:
    5161 add tunable for number of metaslabs per vdev
MFC r272510: MFV r272498:

Add a new sysctl, vfs.zfs.vol.unmap_enabled, which allows the system
administrator to toggle whether ZFS should ignore UNMAP requests.

Illumos issue:
    5149 zvols need a way to ignore DKIOCFREE
Merge remote-tracking branch 'freebsd/stable/10' into hardened/10/aslr