Codechange: C++-ify lists for SaveLoad (#9323)

Basically, this changes "SaveLoad *" to either:
1) "SaveLoadTable" if a list of SaveLoads was meant
2) "SaveLoad &" if a single entry was meant

As added bonus, this removes SL_END / SLE_END / SLEG_END. This
also adds core/span.hpp, a "std::span"-lite.

Fix: Add virtual destructor to link graph Path.

Classes derived from Path were freed through base class pointer, but no virtual destructor was present.

Change: [Linkgraph] Allow job threads to be aborted early when clearing schedule (#8416)

When link graph jobs are cleared due to abandoning the game or exiting,
flag the job as aborted.
The link graph job running in a separate thread checks the aborted flag
periodically and terminates processing early if set.
This reduces the delay at game abandon or exit if a long-running job
would otherwise still be running.

Change: [Linkgraph] Pause the game when linkgraph jobs lag (#6470)

Check if the job is still running two date fract ticks before it is due
to join, and if so pause the game until its done.
When loading a game, check if the game would block immediately due to
a job which is scheduled to be joined within two date fract ticks,
and if so pause the game until its done.
This avoids the main thread being blocked on a thread join, which appears
to the user as if the game is unresponsive, as the UI does not repaint
and cannot be interacted with.
Show if pause is due to link graph job in status bar, update network
messages.
This does not apply for network clients.

Codechange: Replace SmallPair with std::pair.

std::pair is already the smallest possible pair, and it already handles non-POD types correctly.

Codechange: Replace custom thread code with C++11 thread objects.

We assume a conforming C++11 compiler environment that has a valid <thread>-header.
Failure to run a real thread is handled gracefully.