Target remote mode fork and exec event documentation

Target remote mode fork and exec event documentation

This patch implements documentation updates for target remote mode fork and
exec events.  A summary of the rationale for the changes made here:

* Connecting to a remote target -- explain that the two protocols exist.

* Connecting in target remote mode -- explain invoking gdbserver for target
  remote mode, and move remote-specific text from original "Connecting to a
  remote target" section.

* Connecting in target extended-remote mode -- promote this section from
  "Using the gdbserver Program | Running gdbserver | Multi-Process Mode for
  gdbserver".  Put it next to the target remote mode section.

* Host and target files -- collect paragraphs dealing with how to locate
  symbol files from original sections "Connecting to a remote target" and
  "Using the gdbserver program | Connecting to gdbserver".

* Steps for connecting to a remote target -- used to be "Using the
  gdbserver program | Connecting to gdbserver"

* Remote connection commands -- used to be the bulk of "Connecting to a
  remote target".  Added "target extended-remote" commands and information.

gdb/ChangeLog:

	* NEWS: Announce fork and exec event support for target remote.

gdb/doc/ChangeLog:

	* gdb.texinfo (Forks): Correct Linux kernel version where
	fork and exec events are supported, add notes about support
	of these events in target remote mode.
	(Connecting): Reorganize and clarify distinctions between
	target remote, extended-remote, and multiprocess.
	Reorganize related text from separate sections into new
	sections.
	(Server): Note effects of target extended-remote mode.
	Delete section on Multi-Process Mode for gdbserver.
	Move some text to "Connecting" node.