This adopts the Rust postfork code, bridging it from C++ exec module.
We use direct function calls for the bridge, rather than cxx/autocxx, so that we
can be sure that no memory allocations or other shenanigans are happening.
This implements the "postfork" code in Rust, including calling fork(),
exec(), and all the bits that have to happen in between. postfork lives
in the fork_exec module.
It is not yet adopted.
This introduces a new module called fork_exec, which will be for posix_spawn,
postfork, and flog_safe - stuff concerned with actually executing binaries,
and error reporting.
Add a FLOG_SAFE! macro which writes errors to the flog fd in an
async-signal-safe way. This implementation differs from the C++ in that we
allow printing integers directly - no requiring them to be converted to a
buffer first.
