Add a "jail" keyword to list the name of a jail rather than its ID.

[jhb]

Test Plan

• created a simple jail and then ran ps
• if I commit this mwlucas@ will stop telling people to mount procfs in his book

[jhb]

Sample output:

> sudo jail -c -n myjail command=/bin/sh
#

Then on the host:

> ps axO jail -J myjail
  PID JAIL   TT  STAT    TIME COMMAND
66392 myjail 36  I+J  0:00.00 /bin/sh