Add ${name}_env and ${name}_prepend.  ${name}_env is an argument list which
will be passed to env(1).  ${name}_prepend is simply prepended to the command
line for $command.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272974 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f