gdb: use get_standard_config_dir when looking for .gdbinit

This commit effectively changes the default location of the .gdbinit
file, while maintaining backward compatibility.

For non Apple hosts the .gdbinit file will now be looked for in the
following locations:

  $XDG_CONFIG_HOME/gdb/gdbinit
  $HOME/.config/gdb/gdbinit
  $HOME/.gdbinit

On Apple hosts the search order is instead:

  $HOME/Library/Preferences/gdb/gdbinit
  $HOME/.gdbinit

I've performed an extensive rewrite of the documentation, moving all
information about initialization files and where to find them into a
new @node, text from other areas has been moved into this one
location, and other areas cross-reference to this new @node as much as
possible.

gdb/ChangeLog:

	* NEWS: Mention changes to config file search path.
	* main.c

gdb/doc/ChangeLog:

	* gdb.texinfo (Mode Options): Descriptions of initialization files
	has been moved to 'Initialization Files'.
	(Startup): Likewise.
	(Initialization Files): New node.
	(gdb man): Update to mention alternative file paths.
	(gdbinit man): Likewise.