cmdgen: add help for the --primes option.

There's always something. I added the actual option, but forgot to
advertise it in the online help.

While I'm here, I've also allowed the word 'proven' as an alternative
spelling for 'provable', because having the approved spellings be
'probable' and 'provable' is just asking for hilarious typos.

cmdgen.c

@@ -142,6 +142,10 @@ void help(void)
            "        specify file containing new key passphrase\n"
            "  --random-device device\n"
            "        specify device to read entropy from (e.g. /dev/urandom)\n"
+           "  --primes <type>      select prime-generation method:\n"
+           "        probable       conventional probabilistic prime finding\n"
+           "        proven         numbers that have been proven to be prime\n"
+           "        proven-even    also try harder for an even distribution\n"
            );
 }
 
@@ -338,10 +342,12 @@ int main(int argc, char **argv)
                                    !strcmp(val, "probabilistic")) {
                             primegen = &primegen_probabilistic;
                         } else if (!strcmp(val, "provable") ||
+                                   !strcmp(val, "proven") ||
                                    !strcmp(val, "simple") ||
                                    !strcmp(val, "maurer-simple")) {
                             primegen = &primegen_provable_maurer_simple;
                         } else if (!strcmp(val, "provable-even") ||
+                                   !strcmp(val, "proven-even") ||
                                    !strcmp(val, "even") ||
                                    !strcmp(val, "complex") ||
                                    !strcmp(val, "maurer-complex")) {