Un-revert and fix kstart stuff

See the two commits:

Revert "Revert "Start processes with kstart5 if available""

This reverts commit 4baa4cba.

We can fix the issue that prompted the reversion in this repo; see next
commit.

Fix launching commands with args using kstart5

By prepending "--" to the command, kstart5 parses it all as one thing,
so additional arguments are executed as expected.

BUG: 433362
FIXED-IN: 5.86

Second commit uses the authorship of the fellow who discovered this fix in https://bugs.kde.org/show_bug.cgi?id=433362#c11 :)

@davidre