servent tests: Fix failure on 32-bit native Windows. * tests/test-servent.c (getservbyname, getservbyport) [_WIN64 && !__CYGWIN__]: Disable signature check. * doc/posix-functions/getservbyname.texi: Document the incompatible __stdcall function signature. * doc/posix-functions/getservbyport.texi: Likewise.