strerror is not (guaranteed) to be thread safe

This package's `Errno` type is using `strerror` to produce error strings. For unknown error numbers `strerror` writes a string into a static buffer and returns it, but whether this static buffer is thread local is unspecified. Using `strerror_r` would guarantee thread safety even for unknown error codes.

Start by locating the Swift `Errno` implementation and its use of `strerror`. Read the platform-specific POSIX error-string APIs, especially `strerror_r`, and check how unknown error numbers are handled. Done means error-string generation no longer relies on a potentially shared static buffer and the existing relevant tests still pass.

Written by the indexing model from the issue text.