Stop remote-fileio.c from throwing from SIGINT handler

Stop remote-fileio.c from throwing from SIGINT handler

This code installs a custom signal handler that throws a quit
exception if remote_fio_no_longjmp is not set.

AFAICS, the only real reason for this might have been to unblock the
ui_file_read call, in remote_fileio_func_read.  But ever since:

  2009-11-13  Daniel Jacobowitz  <dan@codesourcery.com>

	* ui-file.c (stdio_file_read): Call gdb_select before read.

at:

  https://sourceware.org/ml/gdb-patches/2009-11/msg00321.html

that call is interruptible.

This is not only useful for switching to native C++ exceptions, but
AFAICS, also fixes a potential mess up of the remote protocol
connection, since there are target_read_memory calls done while
remote_fio_no_longjmp is clear.  If the user presses ctrl-c while GDB
is sending or receiving a packet, we'll stop the communication
immediately, at a point where it isn't safe.

gdbserver doesn't support the File I/O remote protocol extension so I
can't test this.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* remote-fileio.c (sigint_fileio_token, remote_fio_no_longjmp):
	Delete.
	(async_remote_fileio_interrupt): Delete.
	(remote_fileio_ctrl_c_signal_handler): Don't call the async signal
	handler.  Instead just always set the ctrl_c flag.
	(remote_fileio_reply): Clear remote_fio_ctrl_c_flag before
	re-enabling the SIGINT handler.
	(remote_fileio_func_open, remote_fileio_func_close)
	(remote_fileio_func_read, remote_fileio_func_write)
	(remote_fileio_func_lseek, remote_fileio_func_rename)
	(remote_fileio_func_unlink, remote_fileio_func_stat)
	(remote_fileio_func_fstat, remote_fileio_func_gettimeofday)
	(remote_fileio_func_isatty, remote_fileio_func_system)
	(remote_fileio_request): Remove references to
	remote_fio_no_longjmp.
	(initialize_remote_fileio): Don't create an async signal handler.