patch 8.0.0804: terminal window functions not yet implemented

Problem:    Terminal window functions not yet implemented.
Solution:   Implement several functions. Add a first test. (Yasuhiro
            Matsumoto, closes #1871)
diff --git a/src/proto/evalfunc.pro b/src/proto/evalfunc.pro
index c673e27..174afb6 100644
--- a/src/proto/evalfunc.pro
+++ b/src/proto/evalfunc.pro
@@ -1,4 +1,5 @@
 /* evalfunc.c */
+buf_T* get_buf_tv(typval_T *tv, int curtab_only);
 char_u *get_function_name(expand_T *xp, int idx);
 char_u *get_expr_name(expand_T *xp, int idx);
 int find_internal_func(char_u *name);