Move init_primes_array out into its own file.

Mostly because I just had a neat idea about how to expose that large
mutable array without it being a mutable global variable: make it a
static in its own module, and expose only a _pointer_ to it, which is
const-qualified.

While I'm there, changed the name to something more descriptive.

smallprimes.c

@@ -0,0 +1,44 @@
+/*
+ * smallprimes.c: implementation of the array of small primes defined
+ * in sshkeygen.h.
+ */
+
+#include <assert.h>
+#include "ssh.h"
+#include "sshkeygen.h"
+
+/* The real array that stores the primes. It has to be writable in
+ * this module, but outside this module, we only expose the
+ * const-qualified pointer 'smallprimes' so that nobody else can
+ * accidentally overwrite it. */
+static unsigned short smallprimes_array[NSMALLPRIMES];
+
+const unsigned short *const smallprimes = smallprimes_array;
+
+void init_smallprimes(void)
+{
+    if (smallprimes_array[0])
+        return;                        /* already done */
+
+    bool A[65536];
+
+    for (size_t i = 2; i < lenof(A); i++)
+        A[i] = true;
+
+    for (size_t i = 2; i < lenof(A); i++) {
+        if (!A[i])
+            continue;
+        for (size_t j = 2*i; j < lenof(A); j += i)
+            A[j] = false;
+    }
+
+    size_t pos = 0;
+    for (size_t i = 2; i < lenof(A); i++) {
+        if (A[i]) {
+            assert(pos < NSMALLPRIMES);
+            smallprimes_array[pos++] = i;
+        }
+    }
+
+    assert(pos == NSMALLPRIMES);
+}