std/sys/fd: Relax READ_LIMIT on Darwin

[morinmorin]

Darwin's read/write syscalls emit EINVAL only when nbyte > INT_MAX. The case nbyte == INT_MAX is valid, so the subtraction (- 1) in

const READ_LIMIT: usize = if cfg!(target_vendor = "apple") {
    libc::c_int::MAX as usize - 1 // <- HERE
} else {
    libc::ssize_t::MAX as usize
};

can be removed.

I tested that the case nbyte == INT_MAX is valid on various versions of macOS, including old one like Mac OS X 10.5.

The man page says:

• read() and pread() will fail if the parameter nbyte exceeds INT_MAX (link: https://keith.github.io/xcode-man-pages/read.2.html)
• write() and pwrite() will fail if the parameter nbyte exceeds INT_MAX (link: https://keith.github.io/xcode-man-pages/write.2.html)

Here are links to Darwin's code:

• [macOS 15.5] https://github.com/apple-oss-distributions/xnu/blob/e3723e1f17661b24996789d8afc084c0c3303b26/bsd/kern/sys_generic.c#L307
• [Mac OS X 10.2] https://github.com/apple/darwin-xnu/blob/d738f900846ed2d5f685e18bf85ce63b0176f61a/bsd/kern/sys_generic.c#L220

Related PR: #38622.

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