[$] The beginning of a process-builder API

The recent discussion on "spawn templates"raised questions about whether it was time to provide an alternative to theclassic Unix fork()/exec() pattern for process creation.One idea that was raised there was to shift the template pattern into aninterface that could be used to efficiently assemble new processes frombare cloth, without duplicating the parent process. Preferably, thatinterface would be able to implement posix_spawn().Li Chen, the author of the spawn-template work, has now responded with a patch series(written with significant LLM assistance) showing what a process-builderAPI for Linux might look like.