patch 8.1.0050: $VIM_TERMINAL is also set when not in a terminal window

Problem:    $VIM_TERMINAL is also set when not in a terminal window.
Solution:   Pass a flag to indicate whether the job runs in a terminal.
diff --git a/src/proto/channel.pro b/src/proto/channel.pro
index e6c9508..5326276 100644
--- a/src/proto/channel.pro
+++ b/src/proto/channel.pro
@@ -66,7 +66,7 @@
 void job_stop_on_exit(void);
 int has_pending_job(void);
 void job_check_ended(void);
-job_T *job_start(typval_T *argvars, char **argv_arg, jobopt_T *opt_arg);
+job_T *job_start(typval_T *argvars, char **argv_arg, jobopt_T *opt_arg, int is_terminal);
 char *job_status(job_T *job);
 void job_info(job_T *job, dict_T *dict);
 void job_info_all(list_T *l);
diff --git a/src/proto/os_unix.pro b/src/proto/os_unix.pro
index 5d90c0e..262a8d5 100644
--- a/src/proto/os_unix.pro
+++ b/src/proto/os_unix.pro
@@ -62,7 +62,7 @@
 void mch_new_shellsize(void);
 void may_send_sigint(int c, pid_t pid, pid_t wpid);
 int mch_call_shell(char_u *cmd, int options);
-void mch_job_start(char **argv, job_T *job, jobopt_T *options);
+void mch_job_start(char **argv, job_T *job, jobopt_T *options, int is_terminal);
 char *mch_job_status(job_T *job);
 job_T *mch_detect_ended_job(job_T *job_list);
 int mch_signal_job(job_T *job, char_u *how);