Illumos #2882, #2883, #2900

2882 implement libzfs_core
2883 changing "canmount" property to "on" should not always remount dataset
2900 "zfs snapshot" should be able to create multiple, arbitrary snapshots at once

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Chris Siden <christopher.siden@delphix.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Bill Pijewski <wdp@joyent.com>
Reviewed by: Dan Kruchinin <dan.kruchinin@gmail.com>
Approved by: Eric Schrock <Eric.Schrock@delphix.com>

References:
  illumos/illumos-gate@4445fffbbb1ea25fd0e9ea68b9380dd7a6709025

Ported-by: Tim Chase <tim@chase2k.com>
Closes #1293

Porting notes:

Add zvol minor device creation to the new zfs_snapshot_nvl function.

Remove the logging of the "release" operation in
dsl_dataset_user_release_sync().  The logging caused a null dereference
because ds->ds_dir is zeroed in dsl_dataset_destroy_sync() and the
logging functions try to get the ds name via the dsl_dataset_name()
function. I've got no idea why this particular code would have worked
in Illumos.  This code has subsequently been completely reworked in
Illumos commit 3b2aab1 (3464 zfs synctask code needs restructuring).

Squash some "may be used uninitialized" warning/erorrs.

Fix some printf format warnings for %lld and %llu.

Apply a few spa_writeable() changes that were made to Illumos in
illumos/illumos-gate.git@cd1c8b8 as part of the 3112, 3113, 3114 and
3115 fixes.

Add a missing call to fnvlist_free(nvl) in log_internal() that was added
in Illumos to fix issue 3085 but couldn't be ported to ZoL at the time
(zfsonlinux/zfs@9e11c73) because it depended on future work.