ci: more GitHub Actions

GitHub Actions is the present and the future.

Provide the base to our docker images and run with the QEMU docker
support optionally.

Add a ThreadSanitizer build

msvc crtdbg -> win32 leakcheck

Add missing worktree_dir check and test case

The win32 leakchecking system is now named win32_leakcheck.  Update the
allocator to match.

Move leak check initialization into git_win32_leakcheck_global_init, and
call it on library initialization.

msvc crtdbg is a mouthfull that is not particularly indicative of what
it does.  Let's rename it to "win32 leakcheck".

Fix the `-DTHREADSAFE=OFF` build

ci: propagate environment variables

ci: supply a token for self-hosted runners

ci: supply a token for self-hosted runners

ci: GitHub Actions for arm64

ci: stop using deprecated set-env in GitHub Actions

(And move the ci scripts into the `ci` directory.)

Deprecate `is_valid_name` functions; replace with `name_is_valid` functions

Fixes #5280

worktree_dir isn't validated when it should be

This change avoids using the `(void)0` construct for some of the mutex
`#define`s, since that makes the "return type" of those "functions" to
be `void` instead of `int`.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Signed-off-by: Sven Strickroth <email@cs-ware.de>