patch 8.0.0949: winpty.dll name is fixed

Problem:    winpty.dll name is fixed.
Solution:   Add the 'winptydll' option. Make the default name depend on
            whether it is a 32-bit or 64-bit build. (idea by Yasuhiro
            Matsumoto, closes #1978)
diff --git a/src/terminal.c b/src/terminal.c
index 149679e..c4fa847 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -38,6 +38,7 @@
  * in tl_scrollback are no longer used.
  *
  * TODO:
+ * - make [range]terminal pipe [range] lines to the terminal
  * - implement term_setsize()
  * - add test for giving error for invalid 'termsize' value.
  * - support minimal size when 'termsize' is "rows*cols".
@@ -2768,11 +2769,15 @@
     /* No need to initialize twice. */
     if (hWinPtyDLL)
 	return 1;
-    /* Load winpty.dll */
-    hWinPtyDLL = vimLoadLib(WINPTY_DLL);
+    /* Load winpty.dll, prefer using the 'winptydll' option, fall back to just
+     * winpty.dll. */
+    if (*p_winptydll != NUL)
+	hWinPtyDLL = vimLoadLib((char *)p_winptydll);
+    if (!hWinPtyDLL)
+	hWinPtyDLL = vimLoadLib(WINPTY_DLL);
     if (!hWinPtyDLL)
     {
-	EMSG2(_(e_loadlib), WINPTY_DLL);
+	EMSG2(_(e_loadlib), *p_winptydll != NUL ? p_winptydll : WINPTY_DLL);
 	return 0;
     }
     for (i = 0; winpty_entry[i].name != NULL