Add "jid" and "jailname" variables to DTrace.

These return the jail ID and jail name for the traced process,
respectively, and are analogous to "zonename" on Solaris/illumos.
"zonename" is now aliased to "jailname".

Also add some stress tests for the new variables.

Submitted by: Domagoj Stolfa <domagoj.stolfa@gmail.com>
Reviewed by: dteske (previous version)
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D13877