QUIT processing w/ explicit throw for gdb_exception_forced_quit

QUIT processing w/ explicit throw for gdb_exception_forced_quit

This commit contains changes which have an explicit throw for
gdb_exception_forced_quit, or, in a couple of cases for gdb_exception,
but with a throw following a check to see if 'reason' is
RETURN_FORCED_QUIT.

Most of these are straightforward - it made sense to continue to allow
an existing catch of gdb_exception to also catch gdb_exception_quit;
in these cases, a catch/throw for gdb_exception_forced_quit was added.

There are two cases, however, which deserve a more detailed
explanation.

1) remote_fileio_request in gdb/remote-fileio.c:

The try block calls do_remote_fileio_request which can (in turn)
call one of the functions in remote_fio_func_map[].  Taking the
first one, remote_fileio_func_open(), we have the following call
path to maybe_quit():

  remote_fileio_func_open(remote_target*, char*)
    -> target_read_memory(unsigned long, unsigned char*, long)
    -> target_read(target_ops*, target_object, char const*, unsigned char*, unsigned long, long)
    -> maybe_quit()

Since there is a path to maybe_quit(), we must ensure that the
catch block is not permitted to swallow a QUIT representing a
SIGTERM.

However, for this case, we must take care not to change the way that
Ctrl-C / SIGINT is handled; we want to send a suitable EINTR reply to
the remote target should that happen.  That being the case, I added a
catch/throw for gdb_exception_forced_quit.  I also did a bit of
rewriting here, adding a catch for gdb_exception_quit in favor of
checking the 'reason' code in the catch block for gdb_exception.

2) mi_execute_command in gdb/mi/mi-main.c:

The try block calls captured_mi_execute_command(); there exists
a call path to maybe_quit():

  captured_mi_execute_command(ui_out*, mi_parse*)
    -> mi_cmd_execute(mi_parse*)
    -> get_current_frame()
    -> get_prev_frame_always_1(frame_info*)
    -> frame_register_unwind_location(frame_info*, int, int*, lval_type*, unsigned long*, int*)
    -> frame_register_unwind(frame_info*, int, int*, int*, lval_type*, unsigned long*, int*, unsigned char*)
    -> value_entirely_available(value*)
    -> value_fetch_lazy(value*)
    -> value_fetch_lazy_memory(value*)
    -> read_value_memory(value*, long, int, unsigned long, unsigned char*, unsigned long)
    -> maybe_quit()

That being the case, we can't allow the exception handler (catch block)
to swallow a gdb_exception_quit for SIGTERM.  However, it does seem
reasonable to output the exception via the mi interface so that some
suitable message regarding SIGTERM might be printed; therefore, I
check the exception's 'reason' field for RETURN_FORCED_QUIT and
do a throw for this case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26761
Tested-by: Tom de Vries <tdevries@suse.de>
Approved-By: Pedro Alves <pedro@palves.net>