rhtooks: multiprocessing: use class-wide lock to prevent race conditions

[rokm]

Use class-wide lock in the FrozenSupportMixIn constructor in multiprocessing run-time hook to prevent race conditions between os.putenv and os.unsetenv calls when processes are spawned concurrently from multiple threads. This essentially serializes process spawning and its associated environment modifications (which are done in main process and inherited by child ones).

Fixes #7410.

Also get rid of redunant check for sys.frozen.

[rokm]

I think a better, long-term solution for this would be to stop clearing the _MEIPASS2 environment variable.

To support spawning other PyInstaller-frozen applications from a frozen application, we should instead store path to PKG archive to another environment variable, and then use that in comparison to determine whether the _MEIPASS2 should be inherited (i.e., re-use the existing temporary directory and files that are extracted there) or re-set (extract files to new temporary directory).

But to avoid breaking the (unlikely?) scenario of having PyInstaller-frozen application with new mechanism run a PyInstaller-frozen application with old mechanism, we would probably have to rename the _MEIPASS2 variable to something else (maybe also something more descriptive, like PYI_TEMPORARY_DIR). So that the application with old mechanism does not end up with _MEIPASS2 set..