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/evalfunc.c b/src/evalfunc.c
index 04e1313..c9d9287 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -787,6 +787,7 @@
 # if defined(FEAT_GUI) || defined(FEAT_TERMGUICOLORS)
     {"term_setansicolors", 2, 2, FEARG_1, f_term_setansicolors},
 # endif
+    {"term_setapi",	2, 2, FEARG_1,	  f_term_setapi},
     {"term_setkill",	2, 2, FEARG_1,	  f_term_setkill},
     {"term_setrestore",	2, 2, FEARG_1,	  f_term_setrestore},
     {"term_setsize",	3, 3, FEARG_1,	  f_term_setsize},