patch 8.0.1743: terminal window options are named inconsistently

Problem:    Terminal window options are named inconsistently.
Solution:   prefix terminal window options with "termwin".  Keep the old names
            for now as an alias.
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index e781a0d..58db1f9 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -130,8 +130,9 @@
       \ 'tagcase': [['smart', 'match'], ['', 'xxx', 'smart,match']],
       \ 'term': [[], []],
       \ 'termguicolors': [[], []],
-      \ 'termsize': [['', '24x80', '0x80', '32x0', '0x0'], ['xxx', '80', '8ax9', '24x80b']],
       \ 'termencoding': [has('gui_gtk') ? [] : ['', 'utf-8'], ['xxx']],
+      \ 'termsize': [['', '24x80', '0x80', '32x0', '0x0'], ['xxx', '80', '8ax9', '24x80b']],
+      \ 'termwinsize': [['', '24x80', '0x80', '32x0', '0x0'], ['xxx', '80', '8ax9', '24x80b']],
       \ 'toolbar': [['', 'icons', 'text'], ['xxx']],
       \ 'toolbariconsize': [['', 'tiny', 'huge'], ['xxx']],
       \ 'ttymouse': [['', 'xterm'], ['xxx']],