MINOR: applet: add a function to reset the svcctx of an applet

MINOR: applet: add a function to reset the svcctx of an applet

The CLI needs to reset the svcctx between commands, and there was nothing
done to handle this. Let's add appctx_reset_svcctx() to do that, it's the
closing equivalent of appctx_reserve_svcctx().

This will have to be backported to 2.6 as it will be used by a subsequent
patch to fix a bug.