patch 9.1.0621: MS-Windows: startup code can be improved

Problem:  MS-Windows: startup code can be improved
Solution: Re-work and optimize win32 startup code
          (Ken Takata)

* Revise the code and reduce #ifdefs.
* For VIMDLL, stop using the default CRT startup code to reduce the file
  size.
  The file size becomes ~130 KB -> ~34 KB on MSVC.
* Update comments. Make them consistent between os_w32dll.c and
  os_w32exe.c.

closes: #15352

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/os_w32dll.c b/src/os_w32dll.c
index e7ba99b..651f96c 100644
--- a/src/os_w32dll.c
+++ b/src/os_w32dll.c
@@ -7,7 +7,7 @@
  * See README.txt for an overview of the Vim source code.
  */
 /*
- * Windows GUI: main program (DLL) entry point:
+ * Windows GUI/Console: main program (DLL) entry point:
  *
  * Ron Aaron <ronaharon@yahoo.com> wrote this and the DLL support code.
  * Adapted by Ken Takata.