sim: don't ignore SIG_TRAP

By ignoring SIG_TRAP, using --debug-break <N> when not connected to
a debugger becomes a no-op.  Apparently this was intended to be a
feature, though the rationale is not clear.

If we don't ignore SIG_TRAP, then using --debug-break <N> when not
connected to a debugger causes the simulation process to terminate
at tick N.  This is occasionally useful, e.g., if you just want to
collect a trace for a specific window of execution then you can combine
this with --debug-start to do exactly that.

In addition to not ignoring the signal, this patch also updates
the --debug-break help message and deletes a handful of unprotected
calls to Debug::breakpoint() that relied on the prior behavior.

src/dev/net/ns_gige.cc

@@ -951,7 +951,6 @@ NSGigE::cpuIntrPost(Tick when)
 
     intrTick = when;
     if (intrTick < curTick()) {
-        Debug::breakpoint();
         intrTick = curTick();
     }
 
@@ -1725,7 +1724,6 @@ NSGigE::txKick()
                             tcp->sum(cksum(tcp));
                             txTcpChecksums++;
                         } else {
-                            Debug::breakpoint();
                             warn_once("TCPPKT set, but not UDP!\n");
                         }
                     }
@@ -1735,7 +1733,6 @@ NSGigE::txKick()
                             ip->sum(cksum(ip));
                             txIpChecksums++;
                         } else {
-                            Debug::breakpoint();
                             warn_once("IPPKT set, but not UDP!\n");
                         }
                     }

src/dev/net/sinic.cc

@@ -527,7 +527,6 @@ Base::cpuIntrPost(Tick when)
 
     intrTick = when;
     if (intrTick < curTick()) {
-        Debug::breakpoint();
         intrTick = curTick();
     }