Could preSpawnProcessConfigurator be made available for posix_spawn OR fork/exec?

[jakepetroules]

Currently, setting preSpawnProcessConfigurator requires pre-fork on Linux, meaning preSpawnProcessConfigurator can't be used to control the posix_spawn path.

On Linux, the signature is:

public var preSpawnProcessConfigurator: (@convention(c) @Sendable () -> Void)? = nil

Could it instead become:

public enum SpawnStrategy {
    case posixSpawn(_ attr: posix_spawnattr_t?, _ fileactions: posix_spawn_file_actions_t?)
    case forkExec
}

public var preSpawnProcessConfigurator: (@convention(c) @Sendable (inout SpawnStrategy) -> Void)? = nil

...and then the configurator could be called for both the posix_spawn and fork/exec paths, without influencing which one is taken.

Bonus points if this could be applied to Darwin as well (removing the forkExec option), to make the PlatformOptions API interface a little more similar.

[jakepetroules]

Because the underlying process spawning implementation could change over time for a given platform (imagine if POSIX standardizes POSIX_SPAWN_CLOEXEC_DEFAULT), and what's allowed in the configurator block has pretty different semantics depending on whether posix_spawn or fork/exec is used (see #132), we could also consider renaming preSpawnProcessConfigurator to postForkProcessConfigurator or similar, to make it clear to users that the latter has a different set of semantics, and to allow preSpawnProcessConfigurator to potentially be introduced for other platforms in the future.

Also, Darwin may use fork and posix_spawn, so that comes into the mix, too.