kinit: Increase environment size limit

start_kdeinit reads its environment over a pipe from start_kdeinit_wrapper. For
security, each environment entry must be smaller than 4kb by default. Qt-based
applications in Nixpkgs may have larger environments, and the recent upgrade to
Plasma 5.17 pushed start_kdeinit_wrapper over the limit. The limit is now
extended to 16kb.

This problem was not detected during testing because the failure is silent:
start_kdeinit will continue with an empty environment. In other circumstances,
this strategy might work, but it does not work on NixOS. This failure is now
treated as a fatal error.

Fixes: #79707