Various improvements to pressure-vessel

• wrap: Add --launcher option

  In the future this option might be used to leverage some of the 'pressure-vessel-launcher' features, like locking specific environment variables.

• wrap: Save original environ before potentially altering it

  If we use the original_environ for all the long-running childs, they'll not be affected even if later on we edit the environ with g_setenv().

• launcher: add support to unset environment variables

  Instead of using env -u VAR to unset a variable now the variables can be passed using the DBUS interface.

/cc @smcv