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/channel.c b/src/channel.c
index 34ee02a..e42c957 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -5144,6 +5144,14 @@
 		memcpy(opt->jo_ansi_colors, rgb, sizeof(rgb));
 	    }
 # endif
+	    else if (STRCMP(hi->hi_key, "term_api") == 0)
+	    {
+		if (!(supported2 & JO2_TERM_API))
+		    break;
+		opt->jo_set2 |= JO2_TERM_API;
+		opt->jo_term_api = tv_get_string_buf_chk(item,
+							 opt->jo_term_api_buf);
+	    }
 #endif
 	    else if (STRCMP(hi->hi_key, "env") == 0)
 	    {