Don't unconditionally set CLOEXEC twice on every fd we open on Linux

Previously, every `open64` was accompanied by a `ioctl(…, FIOCLEX)`,
because some old Linux version would ignore the `O_CLOEXEC` flag we pass
to the `open64` function.

Now, we check whether the `CLOEXEC` flag is set on the first file we
open – if it is, we won't do extra syscalls for every opened file. If it
is not set, we fall back to the old behavior of unconditionally calling
`ioctl(…, FIOCLEX)` on newly opened files.

On old Linuxes, this amounts to one extra syscall per process, namely
the `fcntl(…, F_GETFD)` call to check the `CLOEXEC` flag.

On new Linuxes, this reduces the number of syscalls per opened file by
one, except for the first file, where it does the same number of
syscalls as before (`fcntl(…, F_GETFD)` to check the flag instead of
`ioctl(…, FIOCLEX)` to set it).

Don't unconditionally set CLOEXEC twice on every fd we open on Linux

Previously, every `open64` was accompanied by a `ioctl(…, FIOCLEX)`,
because some old Linux version would ignore the `O_CLOEXEC` flag we pass
to the `open64` function.

Now, we check whether the `CLOEXEC` flag is set on the first file we
open – if it is, we won't do extra syscalls for every opened file. If it
is not set, we fall back to the old behavior of unconditionally calling
`ioctl(…, FIOCLEX)` on newly opened files.

On old Linuxes, this amounts to one extra syscall per process, namely
the `fcntl(…, F_GETFD)` call to check the `CLOEXEC` flag.

On new Linuxes, this reduces the number of syscalls per opened file by
one, except for the first file, where it does the same number of
syscalls as before (`fcntl(…, F_GETFD)` to check the flag instead of
`ioctl(…, FIOCLEX)` to set it).

Don't unconditionally set CLOEXEC twice on every fd we open on Linux

Previously, every `open64` was accompanied by a `ioctl(…, FIOCLEX)`,
because some old Linux version would ignore the `O_CLOEXEC` flag we pass
to the `open64` function.

Now, we check whether the `CLOEXEC` flag is set on the first file we
open – if it is, we won't do extra syscalls for every opened file. If it
is not set, we fall back to the old behavior of unconditionally calling
`ioctl(…, FIOCLEX)` on newly opened files.

On old Linuxes, this amounts to one extra syscall per process, namely
the `fcntl(…, F_GETFD)` call to check the `CLOEXEC` flag.

On new Linuxes, this reduces the number of syscalls per opened file by
one, except for the first file, where it does the same number of
syscalls as before (`fcntl(…, F_GETFD)` to check the flag instead of
`ioctl(…, FIOCLEX)` to set it).

Rollup of 17 pull requests

Successful merges:

 - #50170 (Implement From for more types on Cow)
 - #50638 (Don't unconditionally set CLOEXEC twice on every fd we open on Linux)
 - #50656 (Fix `fn main() -> impl Trait` for non-`Termination` trait)
 - #50669 (rustdoc: deprecate `#![doc(passes, plugins, no_default_passes)]`)
 - #50726 (read2: Use inner function instead of closure)
 - #50728 (Fix rustdoc panic with `impl Trait` in type parameters)
 - #50736 (env: remove unwrap in examples in favor of try op)
 - #50740 (Remove LazyBTreeMap.)
 - #50752 (Add missing error codes in libsyntax-ext asm)
 - #50779 (Make mutable_noalias and arg_align_attributes be tracked)
 - #50787 (Fix run-make wasm tests)
 - #50788 (Fix an ICE when casting a nonexistent const)
 - #50789 (Ensure libraries built in stage0 have unique metadata)
 - #50793 (tidy: Add a check for empty UI test files)
 - #50797 (fix a typo in signed-integer::from_str_radix())
 - #50808 (Stabilize num::NonZeroU*)
 - #50809 (GitHub: Stop treating Cargo.lock as a generated file.)

Failed merges: