patch 8.0.0846: cannot get the name of the pty of a job

Problem:    Cannot get the name of the pty of a job.
Solution:   Add the "tty" entry to the job info. (Ozaki Kiichi, closes #1920)
            Add the term_gettty() function.

src/evalfunc.c

@@ -838,6 +838,7 @@ static struct fst
     {"term_getsize",	1, 1, f_term_getsize},
     {"term_getstatus",	1, 1, f_term_getstatus},
     {"term_gettitle",	1, 1, f_term_gettitle},
+    {"term_gettty",	1, 1, f_term_gettty},
     {"term_list",	0, 0, f_term_list},
     {"term_scrape",	1, 2, f_term_scrape},
     {"term_sendkeys",	2, 2, f_term_sendkeys},