MFV r254748:

Fix memory leak in libzfs's iter_dependents_cb().

Illumos ZFS issues:
  4061 libzfs: memory leak in iter_dependents_cb()
Grow some spares in struct vfsops.

This should hopefully prevent ABI breakage
on adding new vfsops in 10.x.
MFV r254749:

Don't hold dd_lock for long by breaking it when not doing dsl_dir
accounting.  It is not necessary to hold the lock while manipulating
the parent's accounting, because there is no interface for userland
to see a consistent picture of both parent and child at the same
time anyway.

Illumos ZFS issues:
  4046 dsl_dataset_t ds_dir->dd_lock is highly contended
MFV r254751:

Don't treat the parameter as a number (pool GUID) when there is
error converting it from string, instead, treat it as the pool
name.

Illumos ZFS issues:
  1765 assert triggered in libzfs_import.c trying to import pool
       name beginning with a number