Move csrand() to a new file csrand.c

A set of APIs similar to arc4random(3) is complex enough to deserve its
own file.

Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: Björn Esser <besser82@fedoraproject.org>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Joseph Myers <joseph@codesourcery.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/prototypes.h

@@ -356,6 +356,9 @@ extern /*@dependent@*/ /*@null@*/struct commonio_entry *__pw_get_head (void);
 extern /*@null@*/ /*@only@*/struct passwd *__pw_dup (const struct passwd *pwent);
 extern void pw_free (/*@out@*/ /*@only@*/struct passwd *pwent);
 
+/* csrand.c */
+unsigned long csrand (void);
+
 /* remove_tree.c */
 extern int remove_tree (const char *root, bool remove_root);