Fix an assert to tolerate spare parents with more than 2 children.

This can occur if a spare is being spared, which would yield three
children: the original pool drive, the previous spare, and the spare
that is replacing it.

MFC after:	1 week
Sponsored by:	Spectra Logic
Affects:	All ZFS versions starting 7 Jun 2006 (illumos 94de1d4c)
MFSpectraBSD:	r668345 on 2013/06/04 17:10:43
zfs_setprop_error(): Handle errno value E2BIG.

This errno value is emitted by dsl_props_set_check() in
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dsl_prop.c, and
is used to mean that the property value is too long.  For the record,
the maximum length is ZAP_MAXVALUELEN, which is 8*1024 bytes.

Instead of claiming an unknown error (and abort()ing), provide
something more specific to the scenario involved.  As far as I
can tell, E2BIG is not emitted for any other scenario.

MFC after:	1 week
Sponsored by:	Spectra Logic
Affects:	All ZFS versions starting 27 Feb 2009 (illumos ccba0801)
		This change modified the value returned by
		dsl_props_set_check(), so that it can distinguish between
		a name that's too long and a value that's too long, but
		libzfs was not updated accordingly.
MFSpectraBSD:	r1051499 on 2014/03/28 11:07:59