patch 8.1.2094: the fileio.c file is too big

Problem:    The fileio.c file is too big.
Solution:   Move buf_write() to bufwrite.c. (Yegappan Lakshmanan,
            closes #4990)
diff --git a/src/option.c b/src/option.c
index be967dc..cd2cdce 100644
--- a/src/option.c
+++ b/src/option.c
@@ -5530,7 +5530,7 @@
  * After copying window options: update variables depending on options.
  */
     void
-after_copy_winopt(win_T *wp)
+after_copy_winopt(win_T *wp UNUSED)
 {
 #ifdef FEAT_LINEBREAK
     briopt_check(wp);
@@ -5852,6 +5852,8 @@
 #endif
 	    buf->b_p_fo = vim_strsave(p_fo);
 	    buf->b_p_flp = vim_strsave(p_flp);
+	    // NOTE: Valgrind may report a bogus memory leak for 'nrformats'
+	    // when it is set to 8 bytes in defaults.vim.
 	    buf->b_p_nf = vim_strsave(p_nf);
 	    buf->b_p_mps = vim_strsave(p_mps);
 #ifdef FEAT_SMARTINDENT