In a review Pedro pointed out that interp::name is intended to be
read-only, and so an accessor would be a better fit.  This patch
renames the field and adds a "name" method that is used instead.

ChangeLog
2018-05-25  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_enable): Update.
	* mi/mi-interp.c (mi_interp::init): Update.
	* interps.h (class interp) <name>: New method.
	<m_name>: Rename from name.
	(~scoped_restore_interp): Update.
	* interps.c (interp::interp): Update.
	(interp_add, interp_set, interp_lookup_existing)
	(current_interp_named_p): Update.