random_r: new module

random_r: new module

* modules/random_r: New file.
* m4/random_r.m4: New file.
* lib/random_r.c: New file, from glibc.
* modules/random_r-tests: New file.
* tests/test-random_r.c: New file.
* lib/stdlib.in.h (srandom_r, initstate_r, setstate_r, random_r): Declare.
(RAND_MAX): Define.
* m4/stdlib_h.m4: Define and AC_SUBST GNULIB_RANDOM_R and HAVE_RANDOM_R.
* modules/stdlib: Substitute them, too.
* MODULES.html.sh (Extra functions based on POSIX:2001) [Misc]: Add it.
* doc/glibc-functions/initstate_r.texi: Mention the new module.
* doc/glibc-functions/random_r.texi: Likewise.
* doc/glibc-functions/setstate_r.texi: Likewise.
* doc/glibc-functions/srandom_r.texi: Likewise.
* config/srclist.txt: Mention it.