cmdgen: option to specify the random number device to use.

E.g. you might pass '--random-device=/dev/urandom'.

Mostly because I got sick of waiting for /dev/random to finish
blocking while I was trying to generate throwaway keys for testing bug
fixes in cmdgen itself. But it might also be useful on systems that
call their random device by a different name that we haven't
encountered.

(Since cmdgen also reads the saved PuTTY random seed file, setting
this option to /dev/zero will not render key generation deterministic.
It's tempting to provide _some_ way to do that, for testing purposes
and clearly marked as dangerous of course, but I think it would take
more faff than this.)

unix/uxgen.c

@@ -3,21 +3,27 @@
  */
 
 #include <stdio.h>
+#include <errno.h>
+
 #include <fcntl.h>
 #include <unistd.h>
 
 #include "putty.h"
 
-char *get_random_data(int len)
+char *get_random_data(int len, const char *device)
 {
     char *buf = snewn(len, char);
     int fd;
     int ngot, ret;
 
-    fd = open("/dev/random", O_RDONLY);
+    if (!device)
+        device = "/dev/random";
+
+    fd = open(device, O_RDONLY);
     if (fd < 0) {
 	sfree(buf);
-	perror("puttygen: unable to open /dev/random");
+	fprintf(stderr, "puttygen: %s: open: %s\n",
+                device, strerror(errno));
 	return NULL;
     }
 
@@ -27,7 +33,8 @@ char *get_random_data(int len)
 	if (ret < 0) {
 	    close(fd);
             sfree(buf);
-	    perror("puttygen: unable to read /dev/random");
+            fprintf(stderr, "puttygen: %s: read: %s\n",
+                    device, strerror(errno));
 	    return NULL;
 	}
 	ngot += ret;