Don't put non-ASCII into installed headers

Move non-ASCII contributor names from installed headers
into contrib.texi when possible, and when it's not (the
copyright notice in sysdeps/unix/sysv/linux/mips/sys/user.h)
go back to ASCIIfied names.  Problem reported by Joseph Myers in:
https://www.sourceware.org/ml/libc-alpha/2019-08/msg00646.html