patch 8.0.1763: :argedit does not reuse an empty unnamed buffer

Problem:    :argedit does not reuse an empty unnamed buffer.
Solution:   Add the BLN_CURBUF flag and fix all the side effects. (Christian
            Brabandt, closes #2713)
diff --git a/src/proto/buffer.pro b/src/proto/buffer.pro
index 8b98843..59bb2c2 100644
--- a/src/proto/buffer.pro
+++ b/src/proto/buffer.pro
@@ -15,6 +15,7 @@
 void do_autochdir(void);
 void no_write_message(void);
 void no_write_message_nobang(buf_T *buf);
+int curbuf_reusable(void);
 buf_T *buflist_new(char_u *ffname, char_u *sfname, linenr_T lnum, int flags);
 void free_buf_options(buf_T *buf, int free_p_ff);
 int buflist_getfile(int n, linenr_T lnum, int options, int forceit);