[gdb] Handle SIGTERM in run_events

While reviewing "catch (...)" uses I came across:
...
  for (auto &item : local)
    {
      try
	{
	  item ();
	}
      catch (...)
	{
	  /* Ignore exceptions in the callback.  */
	}
    }
...

This means that when an item throws a gdb_exception_forced_quit,
the exception is ignored and following items are executed.

Fix this by handling gdb_exception_forced_quit explicity, and immediately
rethrowing it.

I wondered about ^C, and couldn't decide whether current behaviour is ok, so
I left this alone, but I made the issue explicit in the source code.

As for the "catch (...)", I think that it should let a non-gdb_exception
propagate, so I've narrowed it to "catch (const gdb_exception &)".

My rationale for this is as follows.

There seem to be a few ways that "catch (...)" is allowed in gdb:
- clean-up and rethrow (basically the SCOPE_EXIT pattern)
- catch and handle an exception from a call into an external c++ library

Since we're dealing with neither of those here, we remove the "catch (...)".

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>