patch 8.0.1609: shell commands in the GUI use a dumb terminal

Problem:    Shell commands in the GUI use a dumb terminal.
Solution:   Add the "!" flag to 'guioptions' to execute system commands in a
            special terminal window.  Only for Unix now.
diff --git a/src/proto/terminal.pro b/src/proto/terminal.pro
index 977537b..0ef7783 100644
--- a/src/proto/terminal.pro
+++ b/src/proto/terminal.pro
@@ -1,4 +1,6 @@
 /* terminal.c */
+void init_job_options(jobopt_T *opt);
+buf_T *term_start(typval_T *argvar, char **argv, jobopt_T *opt, int flags);
 void ex_terminal(exarg_T *eap);
 int term_write_session(FILE *fd, win_T *wp);
 int term_should_restore(buf_T *buf);