refactor: I/O safety for unistd.rs #1206

Job	Run time
macos	3m 3s
linux_native_builds (x86_64-unknown-linux-gnu)	2m 37s
macos-aarch64	1m 48s
linux_native_builds (x86_64-unknown-linux-musl)	2m 28s
minver	14s
rust_stable	1m 28s
rustfmt	5s
tier3 (x86_64-unknown-dragonfly)	58s
tier3 (x86_64-unknown-openbsd)	57s
redox	34s
cross (arm-unknown-linux-gnueabi)	1m 40s
cross_compiling (aarch64-linux-android)	30s
cross (armv7-unknown-linux-gnueabihf)	1m 37s
cross_compiling (arm-linux-androideabi)	31s
cross (i686-unknown-linux-gnu)	1m 40s
cross_compiling (armv7-linux-androideabi)	33s
cross (i686-unknown-linux-musl)	1m 42s
cross_compiling (i686-linux-android)	33s
cross (mips-unknown-linux-gnu)	1m 44s
cross_compiling (x86_64-linux-android)	32s
cross (mips64-unknown-linux-gnuabi64)	1m 39s
cross_compiling (arm-unknown-linux-musleabi)	42s
cross (mips64el-unknown-linux-gnuabi64)	1m 46s
cross_compiling (x86_64-unknown-fuchsia)	24s
cross (mipsel-unknown-linux-gnu)	1m 51s
cross_compiling (x86_64-unknown-illumos)	28s
cross (powerpc64le-unknown-linux-gnu)	1m 39s
cross_compiling (aarch64-apple-ios, true)	28s
cross_compiling (powerpc64-unknown-linux-gnu)	32s
cross_compiling (s390x-unknown-linux-gnu)	36s
cross_compiling (x86_64-unknown-linux-gnux32)	37s
cross_compiling (x86_64-unknown-netbsd)	28s
	36m 24s

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