Add get_standard_config_dir function

This adds a new get_standard_config_dir, which returns the name of the
configuration directory.  In XDG, this is ~/.config/gdb/.  Future
patches will make use of this.

2020-07-05  Tom Tromey  <tom@tromey.com>

	* pathstuff.h (get_standard_config_dir): Declare.
	* pathstuff.cc (get_standard_config_dir): New function.