pty: Add a spwan flag to not inherit the parent environment

In gnome-terminal we don't want the gnome-terminal-server's environment
to pollute the child environment, so add the VTE_SPAWN_NO_PARENT_ENVV flag
that when passed to vte_terminal_fork_command_full() makes it *not*
merge the parent's environment.
prompt: Return 0 instead of last exit status