Rollup merge of #39514 - tbu-:pr_less_syscalls_fd, r=alexcrichton
Rollup merge of #39514 - tbu-:pr_less_syscalls_fd, r=alexcrichton

Use less syscalls in `FileDesc::set_{nonblocking,cloexec}`

Only set the flags if they differ from what the OS reported, use
`FIONBIO` to atomically set the non-blocking IO flag on Linux.

graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust