patch 8.1.2080: the terminal API is limited and can't be disabled

Problem:    The terminal API is limited and can't be disabled.
Solution:   Add term_setapi() to set the function prefix. (Ozaki Kiichi,
            closes #2907)
diff --git a/src/structs.h b/src/structs.h
index e3c73cd..9d2ec16 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1938,6 +1938,7 @@
 #define JO2_ANSI_COLORS	    0x8000	// "ansi_colors"
 #define JO2_TTY_TYPE	    0x10000	// "tty_type"
 #define JO2_BUFNR	    0x20000	// "bufnr"
+#define JO2_TERM_API	    0x40000	// "term_api"
 
 #define JO_MODE_ALL	(JO_MODE + JO_IN_MODE + JO_OUT_MODE + JO_ERR_MODE)
 #define JO_CB_ALL \
@@ -2007,6 +2008,8 @@
     long_u	jo_ansi_colors[16];
 # endif
     int		jo_tty_type;	    // first character of "tty_type"
+    char_u	*jo_term_api;
+    char_u	jo_term_api_buf[NUMBUFLEN];
 #endif
 } jobopt_T;