Clarify SIGHUP behavior

The combination of marking it as ACT_IGNORE and osl_Signal_Terminate was
strange, as osl_Signal_Terminate caused SalMainPipeExchangeSignal_impl
(desktop/source/app/officeipcthread.cxx) to call DisableOfficeIPCThread, but
ACT_IGNORE (instead of ACT_EXIT, like other signals marked osl_Signal_Terminate)
caused the process to continue running.  The net effect was that calling an
additional soffice instance after a SIGHUP would no longer connect to the first
instance, but instead would lead to two soffice instances running simultaneously
on the same UserInstallation.

Marking SIGHUP as ACT_HIDE most closely resembles the old behavior (of having
~no effect).  Other options would be to mark it as ACT_IGNORE and
osl_Signal_System or as ACT_EXIT and osl_Signal_Terminate, which would both
result in instant termination.  I do not know if that wouldn't actually be
better.

Change-Id: Id1ee7c1108765301e6010e0e05d6c1ff5894a414

sal/osl/unx/signal.c

@@ -105,7 +105,7 @@ static struct SignalAction
     void (*Handler)(int);
 } Signals[] =
 {
-    { SIGHUP,    ACT_IGNORE, NULL },    /* hangup */
+    { SIGHUP,    ACT_HIDE, NULL },    /* hangup */
     { SIGINT,    ACT_EXIT,   NULL },    /* interrupt (rubout) */
     { SIGQUIT,   ACT_EXIT,  NULL },    /* quit (ASCII FS) */
     { SIGILL,    ACT_SYSTEM,  NULL },    /* illegal instruction (not reset when caught) */
@@ -973,7 +973,6 @@ void SignalHandlerFunction(int Signal)
         case SIGINT:
         case SIGTERM:
         case SIGQUIT:
-        case SIGHUP:
             Info.Signal = osl_Signal_Terminate;
             break;