Issue Description:

Writing code that reads Glibc.errno or Darwin.errno generates concurrency warnings when compiled in -warn-concurrency mode:

/code/Sources/NIOCore/SystemCallHelpers.swift:66:23: warning: reference to var 'errno' is not concurrency-safe because it involves shared mutable state
            let err = errno
                      ^
Glibc.errno:1:12: note: var declared here
public var errno: Int32 { get set }
           ^

This warning is incorrect: errno is thread-local and so there is no concurrency failure here. Note that because it's thread-local it's not always safe to use errno in an actor context, but this warning is emitted even in non-actor contexts.