patch 8.2.3780: ":cd" works differently on MS-Windows

Problem:    ":cd" works differently on MS-Windows.
Solution:   Add the 'cdhome' option. (closes #9324)
diff --git a/src/option.h b/src/option.h
index fc81245..49d869b 100644
--- a/src/option.h
+++ b/src/option.h
@@ -1094,6 +1094,7 @@
 EXTERN int	p_wa;		// 'writeany'
 EXTERN int	p_wb;		// 'writebackup'
 EXTERN long	p_wd;		// 'writedelay'
+EXTERN int	p_cdh;		// 'cdhome'
 
 /*
  * "indir" values for buffer-local options.