Disable Lua posix.fork() and friends on v6 packages

[pmatilai]

As discussed in #3270, we can't actually remove the support for posix.fork() and the associated functions even in v6, because the handful of users are actually key packages in distros, and removing the functionality would be a far, far bigger compatibility break than v6 format, and that makes no sense whatsoever.

That PR adds poor mans dynamic symbol versioning mechanism for our Lua APIs, for 6.0 we should extend that to entirely disable these symbols for v6 packages, so the behavior for using these in 6.0 would be:

• packages built with < 4.20 install with no warnings
• v4 packages built with >= 4.20 issue thd deprecation warnings
• v6 packages error out when using these functions