Change session exit detection

Previously we waited for all opened file descriptors to the terminal
to be closed. This caused problem when e.g. running "sleep 900 &"
and then exiting the shell, with sleep keeping the session alive and
had to be killed manually (killing the process group did not help -
the shell had already exited and was in zombie state). This is also
what most other terminal emulators do.

Relatedly, switch to sending SIGKILL to force quit a session instead
of SIGHUP, since SIGHUP can be ignored.

app/src/main/jni/termux.c

@@ -208,11 +208,6 @@ JNIEXPORT int JNICALL Java_com_termux_terminal_JNI_waitFor(JNIEnv* TERMUX_UNUSED
     }
 }
 
-JNIEXPORT void JNICALL Java_com_termux_terminal_JNI_hangupProcessGroup(JNIEnv* TERMUX_UNUSED(env), jclass TERMUX_UNUSED(clazz), jint procId)
-{
-    killpg(procId, SIGHUP);
-}
-
 JNIEXPORT void JNICALL Java_com_termux_terminal_JNI_close(JNIEnv* TERMUX_UNUSED(env), jclass TERMUX_UNUSED(clazz), jint fileDescriptor)
 {
     close(fileDescriptor);