patch 8.0.0728: the terminal structure is never freed

Problem:    The terminal structure is never freed.
Solution:   Free the structure and unreference what it contains.
diff --git a/src/proto/terminal.pro b/src/proto/terminal.pro
index e89b0a2..58eb77f 100644
--- a/src/proto/terminal.pro
+++ b/src/proto/terminal.pro
@@ -1,5 +1,6 @@
 /* terminal.c */
 void ex_terminal(exarg_T *eap);
+void free_terminal(term_T *term);
 void write_to_term(buf_T *buffer, char_u *msg, channel_T *channel);
 void term_update_window(win_T *wp);
 void terminal_loop(void);