MFV r323527: 5815 libzpool's panic function doesn't set global panicstr, ::status not as useful

illumos/illumos-gate@fae6347731c9d3f46b26338313b0422927f29cf6
https://github.com/illumos/illumos-gate/commit/fae6347731c9d3f46b26338313b0422927f29cf6

https://www.illumos.org/issues/5815

When panic() is called from within ztest, the mdb ::status command isn't as
useful as it could be since the global panicstr variable isn't updated. We
should modify the function to make sure panicstr is set, so ::status can
present the error message just like it does on a failed assertion.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>
Reviewed by: Gordon Ross <gordon.ross@nexenta.com>
Reviewed by: Rich Lowe <richlowe@richlowe.net>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Prakash Surya <prakash.surya@delphix.com>
MFC after: 4 weeks