Rename most of the platform source files.

This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.

unix/keygen-noise.c

@@ -0,0 +1,64 @@
+/*
+ * uxgen.c: Unix implementation of get_heavy_noise() from cmdgen.c.
+ */
+
+#include <stdio.h>
+#include <errno.h>
+
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "putty.h"
+
+char *get_random_data(int len, const char *device)
+{
+    char *buf = snewn(len, char);
+    int fd;
+    int ngot, ret;
+
+    if (!device) {
+        static const char *const default_devices[] = {
+            "/dev/urandom", "/dev/random"
+        };
+        size_t i;
+
+        for (i = 0; i < lenof(default_devices); i++) {
+            if (access(default_devices[i], R_OK) == 0) {
+                device = default_devices[i];
+                break;
+            }
+        }
+
+        if (!device) {
+            sfree(buf);
+            fprintf(stderr, "puttygen: cannot find a readable "
+                    "random number source; use --random-device\n");
+            return NULL;
+        }
+    }
+
+    fd = open(device, O_RDONLY);
+    if (fd < 0) {
+        sfree(buf);
+        fprintf(stderr, "puttygen: %s: open: %s\n",
+                device, strerror(errno));
+        return NULL;
+    }
+
+    ngot = 0;
+    while (ngot < len) {
+        ret = read(fd, buf+ngot, len-ngot);
+        if (ret < 0) {
+            close(fd);
+            sfree(buf);
+            fprintf(stderr, "puttygen: %s: read: %s\n",
+                    device, strerror(errno));
+            return NULL;
+        }
+        ngot += ret;
+    }
+
+    close(fd);
+
+    return buf;
+}