removed styling of dt tages, which would make them look like keys and
added <kbd> tag inside of dt tag.
Added css style for kbd and removed some stylings for dt

To workaround a bug in glibc <= 2.26 lookup_host() calls res_init()
based on the glibc version detected at runtime. While this avoids
calling res_init() on platforms where it's not required we will still
end up linking against the symbol.

This causes an issue on macOS where res_init() is implemented in a
separate library (libresolv.9.dylib) from the main libc. While this is
harmless for standalone programs it becomes a problem if Rust code is
statically linked against another program. If the linked program doesn't
already specify -lresolv it will cause the link to fail. This is
captured in issue rust-lang#46797

Fix this by hooking in to the glibc workaround in `cvt_gai` and only
activating it for the "gnu" environment on Unix This should include all
glibc platforms while excluding musl, windows-gnu, macOS, FreeBSD, etc.

This has the side benefit of removing the #[cfg] in sys_common; only
unix.rs has code related to the workaround now.

…more performant variant.

We intended to do this quite a while ago but it snuck through.

…laumeGomez

add kbd style tag to main.css in rustdoc

Added css style for kbd tags so they actually look like keys.
Result preview and discussion was going on in rust-lang#46900 .

…nix, r=alexcrichton

Only link res_init() on GNU/*nix

To workaround a bug in glibc <= 2.26 lookup_host() calls res_init() based on the glibc version detected at runtime. While this avoids calling res_init() on platforms where it's not required we will still end up linking against the symbol.

This causes an issue on macOS where res_init() is implemented in a separate library (libresolv.9.dylib) from the main libc. While this is harmless for standalone programs it becomes a problem if Rust code is statically linked against another program. If the linked program doesn't already specify -lresolv it will cause the link to fail. This is captured in issue rust-lang#46797

Fix this by hooking in to the glibc workaround in `cvt_gai` and only activating it for the "gnu" environment on Unix This should include all glibc platforms while excluding musl, windows-gnu, macOS, FreeBSD, etc.

This has the side benefit of removing the #[cfg] in sys_common; only unix.rs has code related to the workaround now.

Before this commit:
```shell
> cat main.rs
use std::net::ToSocketAddrs;

#[no_mangle]
pub extern "C" fn resolve_test() -> () {
    let addr_list = ("google.com.au", 0).to_socket_addrs().unwrap();
    println!("{:?}", addr_list);
}
> rustc --crate-type=staticlib main.rs
> clang libmain.a test.c -o combined
Undefined symbols for architecture x86_64:
  "_res_9_init", referenced from:
      std::net::lookup_host::h93c17fe9ad38464a in libmain.a(std-826c8d3b356e180c.std0.rcgu.o)
ld: symbol(s) not found for architecture x86_64
clang-5.0: error: linker command failed with exit code 1 (use -v to see invocation)
```

Afterwards:
```shell
> rustc --crate-type=staticlib main.rs
> clang libmain.a test.c -o combined
> ./combined
IntoIter([V4(172.217.25.131:0)])
```

Fixes  rust-lang#46797

Fix off-by-one spans in MIR borrowck errors

Fixes rust-lang#46885.

r? @nikomatsakis

…eveklabnik

add Rust By Example to the bookshelf

cc rust-lang#46194

With rust-lang#46196 freshly merged, we should add a link to the main docs distribution so people can find it! We discussed this at the docs team meeting today and decided to go ahead with adding it to the bookshelf.

Deprecate std::net::lookup_host

We intended to do this quite a while ago but it snuck through.

r? @alexcrichton

Add E0659 for ambiguous names

Still on the tracks of the "no error without error code" road.

add target/ to ignored tidy dirs

Sometimes you get a target directory from running cargo in the rust repo (the root is `src/`), and it contains generated files. Just whitelist it since it causes tidy to spew warnings uncontrollably.

Fix the "Github - About Pull Requests" link in CONTRIBUTING.md

Previously the text that is supposed to link to the Github Pull Requests page, instead linked to the same file (CONTRIBUTING.md).