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.
