Whitespace rationalisation of entire code base.

The number of people has been steadily increasing who read our source
code with an editor that thinks tab stops are 4 spaces apart, as
opposed to the traditional tty-derived 8 that the PuTTY code expects.

So I've been wondering for ages about just fixing it, and switching to
a spaces-only policy throughout the code. And I recently found out
about 'git blame -w', which should make this change not too disruptive
for the purposes of source-control archaeology; so perhaps now is the
time.

While I'm at it, I've also taken the opportunity to remove all the
trailing spaces from source lines (on the basis that git dislikes
them, and is the only thing that seems to have a strong opinion one
way or the other).
    
Apologies to anyone downstream of this code who has complicated patch
sets to rebase past this change. I don't intend it to be needed again.

unix/uxsel.c

@@ -1,6 +1,6 @@
 /*
  * uxsel.c
- * 
+ *
  * This module is a sort of all-purpose interchange for file
  * descriptors. At one end it talks to uxnet.c and pty.c and
  * anything else which might have one or more fds that need
@@ -17,7 +17,7 @@
 
 struct fd {
     int fd;
-    int rwx;			       /* 4=except 2=write 1=read */
+    int rwx;                           /* 4=except 2=write 1=read */
     uxsel_callback_fn callback;
     uxsel_id *id;                      /* for uxsel_input_remove */
 };
@@ -29,9 +29,9 @@ static int uxsel_fd_cmp(void *av, void *bv)
     struct fd *a = (struct fd *)av;
     struct fd *b = (struct fd *)bv;
     if (a->fd < b->fd)
-	return -1;
+        return -1;
     if (a->fd > b->fd)
-	return +1;
+        return +1;
     return 0;
 }
 static int uxsel_fd_findcmp(void *av, void *bv)
@@ -39,9 +39,9 @@ static int uxsel_fd_findcmp(void *av, void *bv)
     int *a = (int *)av;
     struct fd *b = (struct fd *)bv;
     if (*a < b->fd)
-	return -1;
+        return -1;
     if (*a > b->fd)
-	return +1;
+        return +1;
     return 0;
 }
 
@@ -69,12 +69,12 @@ void uxsel_set(int fd, int rwx, uxsel_callback_fn callback)
     uxsel_del(fd);
 
     if (rwx) {
-	newfd = snew(struct fd);
-	newfd->fd = fd;
-	newfd->rwx = rwx;
-	newfd->callback = callback;
-	newfd->id = uxsel_input_add(fd, rwx);
-	add234(fds, newfd);
+        newfd = snew(struct fd);
+        newfd->fd = fd;
+        newfd->rwx = rwx;
+        newfd->callback = callback;
+        newfd->id = uxsel_input_add(fd, rwx);
+        add234(fds, newfd);
     }
 }
 
@@ -82,16 +82,16 @@ void uxsel_del(int fd)
 {
     struct fd *oldfd = find234(fds, &fd, uxsel_fd_findcmp);
     if (oldfd) {
-	if (oldfd->id)
+        if (oldfd->id)
             uxsel_input_remove(oldfd->id);
-	del234(fds, oldfd);
-	sfree(oldfd);
+        del234(fds, oldfd);
+        sfree(oldfd);
     }
 }
 
 /*
  * And here is the interface to select-functionality-supplying
- * modules. 
+ * modules.
  */
 
 int next_fd(int *state, int *rwx)
@@ -99,10 +99,10 @@ int next_fd(int *state, int *rwx)
     struct fd *fd;
     fd = index234(fds, (*state)++);
     if (fd) {
-	*rwx = fd->rwx;
-	return fd->fd;
+        *rwx = fd->rwx;
+        return fd->fd;
     } else
-	return -1;
+        return -1;
 }
 
 int first_fd(int *state, int *rwx)