Fix fcntl for FreeBSD platform

[AlexanderThaller]

When compiling the following code with nix 0.19.1 under FreeBSD 12.2:

use std::os::unix::io::AsRawFd;

fn main() {
    let f = std::fs::File::create("/tmp/testfile").unwrap();

    nix::fcntl::posix_fadvise(
        f.as_raw_fd(),
        0,
        0,
        nix::fcntl::PosixFadviseAdvice::POSIX_FADV_NOREUSE,
    ).unwrap();
}

I get the following error:

error[E0433]: failed to resolve: could not find `PosixFadviseAdvice` in `fcntl`
  --> src/main.rs:10:21
   |
10 |         nix::fcntl::PosixFadviseAdvice::POSIX_FADV_NOREUSE,
   |                     ^^^^^^^^^^^^^^^^^^ could not find `PosixFadviseAdvice` in `fcntl`

error[E0425]: cannot find function `posix_fadvise` in module `nix::fcntl`
 --> src/main.rs:6:17
  |
6 |     nix::fcntl::posix_fadvise(
  |                 ^^^^^^^^^^^^^ not found in `nix::fcntl`

error: aborting due to 2 previous errors

Checking the documentation I noticed the documentation for the FreeBSD platform was missing:

https://docs.rs/nix/0.19.1/x86_64-unknown-freebsd/nix/?search=PosixFadviseAdvice

Checking the code I noticed that target_env was used instead of target_os:

• https://doc.rust-lang.org/reference/conditional-compilation.html#target_env
• https://doc.rust-lang.org/reference/conditional-compilation.html#target_os

Switching to target_os fixed the compilation errors. I also ran the tests with the fix and they seemed to be fine:

test result: ok. 68 passed; 0 failed; 14 ignored; 0 measured; 0 filtered out

I hope this makes sense to fix.

[asomers]

Nice catch!

bors r+

[bors]

Build succeeded:

• Linux MIPS
• Redox x86_64
• Linux aarch64
• Linux i686 musl
• Linux arm gnueabi
• Minver
• Linux armv7 gnueabihf
• Android i686
• Android armv7
• FreeBSD amd64 & i686
• iOS
• Android arm
• Linux MIPS64 el
• OSX i686
• Rust Stable
• OSX x86_64
• Linux i686
• Linux MIPS64
• Linux mipsel
• Linux powerpc64le
• Android aarch64
• Linux arm-musleabi
• Linux x86_64 musl
• Linux x86_64
• Android x86_64
• Linux s390x
• Linux powerpc
• NetBSD x86_64