Document pseudo-terminal and interrupting changes

Document pseudo-terminal and interrupting changes

This documents changes done in previous patches:

 - the fact that on GNU/Linux, GDB creates a pseudo-terminal for the
   inferior instead of juggling terminal settings.

 - That when the inferior and GDB share the terminal, you can't
   interrupt some programs with Ctrl-C.

 - That on GNU/Linux, you may get "Program stopped." instead of
   "Program received SIGINT" in response to Ctrl-C.

 - That run+detach may result in the program dying with SIGHUP.

I was surprised that we do not currently have a node/section
specifically to talk about interrupting programs.  Thus I've added a
new "Interrupting" section under the "Stopping and Continuing"
chapter, with some xrefs to other sections.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* NEWS: Document pseudo-terminal, "tty /dev/tty" and Ctrl-C/SIGINT
	changes.  Document "set/show debug managed-tty".

gdb/doc/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* gdb.texinfo (Input/Output): Document that GDB may start a
	program associated with a pseudo-terminal.  Document "tty
	/dev/tty".  Document "set/show debug managed-tty".
	(Attach): Document what happens on run+detach on systems where GDB
	creates a pseudo-terminal for the inferior.
	(Stopping and Continuing): Add new Interrupting node.
	(Background Execution): Add anchor.
	(Features for Debugging MS Windows PE Executables): Add anchor.

Change-Id: I267a0f9300c7ac4d2e7f14a9ba8eabc1eafcc5a7