Pre-select the last sent signal in SignalsPanel

Instead of pre-selecting SIGTERM every time, select the signal last
send in the same htop session.

Closes: #862

Action.c

@@ -344,9 +344,12 @@ static Htop_Reaction actionKill(State* st) {
    if (Settings_isReadonly())
       return HTOP_OK;
 
-   Panel* signalsPanel = SignalsPanel_new();
+   static int preSelectedSignal = SIGNALSPANEL_INITSELECTEDSIGNAL;
+
+   Panel* signalsPanel = SignalsPanel_new(preSelectedSignal);
    const ListItem* sgn = (ListItem*) Action_pickFromVector(st, signalsPanel, 14, true);
    if (sgn && sgn->key != 0) {
+      preSelectedSignal = sgn->key;
       Panel_setHeader((Panel*)st->mainPanel, "Sending...");
       Panel_draw((Panel*)st->mainPanel, false, true, true, State_hideFunctionBar(st));
       refresh();

SignalsPanel.c

@@ -18,15 +18,14 @@ in the source distribution for its full text.
 #include "XUtils.h"
 
 
-Panel* SignalsPanel_new() {
+Panel* SignalsPanel_new(int preSelectedSignal) {
    Panel* this = Panel_new(1, 1, 1, 1, Class(ListItem), true, FunctionBar_newEnterEsc("Send   ", "Cancel "));
-   const int defaultSignal = SIGTERM;
    int defaultPosition = 15;
    unsigned int i;
    for (i = 0; i < Platform_numberOfSignals; i++) {
       Panel_set(this, i, (Object*) ListItem_new(Platform_signals[i].name, Platform_signals[i].number));
       // signal 15 is not always the 15th signal in the table
-      if (Platform_signals[i].number == defaultSignal) {
+      if (Platform_signals[i].number == preSelectedSignal) {
          defaultPosition = i;
       }
    }

SignalsPanel.h

@@ -7,6 +7,8 @@ Released under the GNU GPLv2+, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include <signal.h>
+
 #include "Panel.h"
 
 
@@ -15,6 +17,8 @@ typedef struct SignalItem_ {
    int number;
 } SignalItem;
 
-Panel* SignalsPanel_new(void);
+#define SIGNALSPANEL_INITSELECTEDSIGNAL SIGTERM
+
+Panel* SignalsPanel_new(int preSelectedSignal);
 
 #endif