New functions to shrink a strbuf.

These are better than my previous approach of just assigning to
sb->len, because firstly they check by assertion that the new length
is within range, and secondly they preserve the invariant that the
byte stored in the buffer just after the length runs out is \0.

Switched to using the new functions everywhere a grep could turn up
opportunities.

windows/winpgnt.c

@@ -678,7 +678,7 @@ static void update_sessions(void)
     sb = strbuf_new();
     while(ERROR_SUCCESS == RegEnumKey(hkey, index_key, buf, MAX_PATH)) {
         if(strcmp(buf, PUTTY_DEFAULT) != 0) {
-            sb->len = 0;
+            strbuf_clear(sb);
             unescape_registry_key(buf, sb);
 
             memset(&mii, 0, sizeof(mii));