select_wrapper_t wraps up the annoying bits of using select(): keeping
track of the max fd, passing null for boring parameters, and
constructing the timeout. Introduce a wrapper struct for this and
replace the existing uses of select() with the wrapper.
This concerns how fish prevents its own fds from interfering with
user-defined fd redirections, like `echo hi >&5`. fish has historically
done this by tracking all user defined redirections when running a job,
and ensuring that pipes are not assigned the same fds. However this is
annoying to pass around - it means that we have to thread user-defined
redirections into pipe creation.
Take a page from zsh and just ensure that all pipes we create have fds in
the "high range," which here means at least 10. The primary way to do this
is via the F_DUPFD_CLOEXEC syscall, which also sets CLOEXEC, so we aren't
invoking additional syscalls in the common case. This will free us from
having to track which fds are in user-defined redirections.
fds.h will centralize logic around working with file descriptors. In
particular it will be the new home for logic around moving fds to high
unused values, replacing the "avoid conflicts" logic.
