Mac: fix Apple Silicon GPU errors after MacOS update

[CharlieFenton]

User boinc_master should have primary group boinc_master. User boincproject should have primary group boincproject.
But updating MacOS to a new version sometimes changes the PrimaryGroupID of users boinc_master and boincproject to staff (20). This seems to be responsible for OpenCL projects failing after a MacOS update with the following error:
UNSUPPORTED (log once): buildComputeProgram: cl2Metal failed <built-in>:1:10: fatal error: cannot open file './opencl-c.h': Permission denied #include "opencl-c.h"

Fixes #5456

This PR makes the following changes:

• Added a check of the Primary Group IDs of users boinc_master and boinc_project along with the other permissions checks when the Mac Manager or client is launched. If incorrect, refuses to run and displays an alert "BOINC ownership or permissions are not set properly; please reinstall BOINC".
• The Mac Installer: always sets Primary Group of users boinc_master boinc_project. (It previously did that only if those users did not already exist.)

[CharlieFenton]

I have built and uploaded BOINC 7.24.3. This is a hot fix for issue #5456. The changes affect only the permissions checks when BOINC is started and the way the BOINC installer sets permissions. It makes no changes to the way BOINC computes or interacts with project servers. Therefore, I have immediately promoted it to recommended version.