Add configure tests and defines for getservbyname_r, getservbyport_r, andgetprotobyname_r. Use these if available, otherwise fallback to thethread-unsafe variants.Add a unit test to exercise getprotobyname, which is currently untested.TODO: - Are there any platforms which define the unsafe variants but not the   re-entrant ones? If not we can simplify the #ifdef hell somewhat. - Do the re-entrant functions have the same signature on all platforms? - These changes follow the existing code's practice: allocate a fixed-size   (and overly large) buffer, and don't properly handle the error case if it is   too small. Should this be fixed? If so should existing code also be fixed?

The libc setpwent, getpwent, and endpwent functions are not thread-safe.Protect them with mutexs in free-threading builds.

The dbm_* functions are not thread-safe, naturally. Add critical sections toprotect their use.

The getlogin function is not thread-safe: replace with getlogin_r whereavailable.Note that this function is untested (unit test is skipped with a note it causedCI failures as behaviour differs between NIX environments).