Convert coreutils' rand*.{c,h,m4} into modules.

Convert coreutils' rand*.{c,h,m4} into modules.

First step: move these files to gl/lib:
* lib/rand-isaac.c, lib/rand-isaac.h
* lib/randint.c, lib/randint.h
* lib/randperm.c, lib/randperm.h
* lib/randread.c, lib/randread.h

Step 2: add modules/rand* and remove now-unneeded .m4 files.
* gl/modules/randint: New file.
* gl/modules/randperm: New file.
* gl/modules/randread: New file.
* m4/randint.m4: Remove file.
* m4/randperm.m4: Remove file.
* m4/randread.m4: Remove file.

Step 3: use the new modules
* bootstrap.conf (gnulib_modules): Add randint and randperm.
* m4/prereq.m4 (gl_RANDINT, gl_RANDREAD, gl_RANDPERM): Don't require;
These have been removed.
(gl_ROOT_DEV_INO): Don't require; already handled via bootstrap.conf.