Add basic form of the bd_swap_swapon function

No checks, just plain 'swapon' call.
Move code for calling utilities and reporting errors to one place

This way it can be reused by various functions in the plugin.
Add NULL as a termination value to the argv arrays

GLib's g_spawn_sync requires the array to be zero-terminated (no way to
determine the end otherwise).
Remove newlines from the swap test outputs

They are already included in the utility's output.
Basic version of the bd_swap_swapoff function
Add the bd_swap_swapstatus function
Add a target to run ipython as root with the library available

This may come handy for testing (on a non-production system!).