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 #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.
The previous workaround for gibc's res_init bug is not thread-safe on
other implementations of libc, and it can cause crashes. Use a runtime
check to make sure we only call res_init when we need to, which is also
when it's safe. See https://github.com/rust-lang/rust/issues/43592.
As suggested in the discussion of PR #43972, std should provide a uniform API to
all platforms. Since there's no networking on L4Re, this now is a module in
`sys::net` providing types and functions/methods returning an error for each
action.
