propose removing strnlen dependency on memchr

[Paul Eggert]

I considered adding the strnlen module to Emacs, but this had
the problem of bringing in the memchr module, which is pretty
heavyweight.  I propose that we remove this dependency, as
the bugs fixed by the memchr module are not tickled by
the gnulib strnlen implementation.

Possibly other dependencies on memchr could be removed too,
but I haven't checked for this.