RSA generation: change exponent to 2^16+1.

This is probably overdue; everyone else seems to have settled on it as
the preferred RSA key exponent for some time. And now that the
descendant of mp_mod_short supports moduli up to 2^32 instead of 2^16,
I can actually add it without the risk of assertion failures during
prime generation.

sshrsag.c

@@ -7,7 +7,7 @@
 #include "ssh.h"
 #include "mpint.h"
 
-#define RSA_EXPONENT 37                /* we like this prime */
+#define RSA_EXPONENT 65537
 
 static void invent_firstbits(unsigned *one, unsigned *two,
                              unsigned min_separation);