Use the ErrorOr pattern with the Core::System wrappers to propagate more
errors from the TerminalWrapper.
The run_command method, when called with WaitForExit::Yes now returns an
error on command failure.
Previously the setup for both the master and slave pseudoterminals was
done in TerminalWrapper::run_command.
This commit separates the relevant logic into
TerminalWrapper::setup_master_pseudoterminal
and TerminalWrapper::setup_slave_pseudoterminal.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
