Complete support for dtrace's -x setenv option.

This allows one to override the environment for processes created with
dtrace -c. By default, the environment is inherited.

This support was originally merged from illumos in r249367 but was lost
when the commit was later reverted and then brought back piecemeal.

Reported by: Samuel Lepetit <slepetit@apple.com>
MFC after: 2 weeks