execute_process: Restore termination of processes on timeout

Since commit 5420639a8d (cmExecuteProcessCommand: Replace cmsysProcess
with cmUVProcessChain, 2023-06-01, v3.28.0-rc1~138^2~8) we have not
actually terminated child processes on an `execute_process` timeout.
Similarly for other migrations from cmsysProcess to cmUVProcessChain.

Teach cmUVProcessChain clients that implement timeouts to actually
terminate remaining child processes when the timeout is reached.

Fixes: #27378