Specify SA_RESTART when registering the SIGWINCH signal handler

Specify SA_RESTART when registering the SIGWINCH signal handler

SA_RESTART allows system calls to be restarted across a signal handler.
By specifying this flag we fix the issue where if the user is being
prompted to answer yes or no, and the terminal gets resized in the
meantime, the prompt will think that the user sent an EOF and so it will
take the default action for that prompt (in the case of the quit prompt,
it will quit GDB).

gdb/ChangeLog:

	* tui/tui-win.c (tui_initialize_win): Specify SA_RESTART when
	registering the signal handler.