[Backtracing] Bail out earlier for privileged binaries on macOS.

[al45tair]

Also remove the code that deals with file descriptors; we will now only start the backtracer for processes that have the get-task-allow entitlement, which means that they've been specifically signed for debugging, and that means that it is no longer critical to ensure that unexpectedly inherited fds are closed.

rdar://137551812

[al45tair]

@swift-ci Please test

[al45tair]

Explanation: This is a belt & braces improvement for the backtracer's security; instead of relying on the swift-backtrace process to check for some of the privileged types of program, we check up front in the runtime. We weren't doing this originally because I was concerned about calling csops from a potentially arbitrary sandbox, but I've been assured that that should be OK, and by checking up front for get-task-allow, we can also be more relaxed about closing file descriptors (since the process is already marked as suitable for debugging, so it's easy to get hold of its fds anyway).
Risk: Low. This just improves the checks that we do to make things even more secure.
Original PR: #77162 + #77188 + #77200
Reviewed by: @mikeash
Resolves: rdar://137551812
Tests: The backtracer's tests exercise this code.

[al45tair]

@swift-ci Please test

[al45tair]

@swift-ci Please test