patch 8.0.0419: test for v:progpath fails on MS-Windows

Problem:    Test for v:progpath fails on MS-Windows.
Solution:   Expand to full path.  Also add ".exe" when the path is an absolute
            path.
diff --git a/src/os_win32.c b/src/os_win32.c
index dd3e6de..7081e03 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -1921,7 +1921,12 @@
 	if (mch_getperm(name) != -1 && !mch_isdir(name))
 	{
 	    if (path != NULL)
-		*path = vim_strsave((char_u *)name);
+	    {
+		if (mch_isFullName(name))
+		    *path = vim_strsave((char_u *)name);
+		else
+		    *path = FullName_save((char_u *)name, FALSE);
+	    }
 	    return TRUE;
 	}
 	return FALSE;