Eliminate the 'the_core_target' global

Eliminate the 'the_core_target' global

(previously called 'core_target', but since renamed because
'core_target' is the name of the target_ops class now.)

This eliminates the "the_core_target" global, as preparation for being
able to have more than one core loaded.  When we get there, we will
instantiate one core_target object per core instead.

Essentially, this replaces the reference to the_core_target in
core_file_command by a reference to core_bfd, which is per
program_space.

Currently, core_file_command calls 'the_core_target->detach()' even if
the core target is not open and pushed on the target stack.  If it is
indeed not open, then the practical effect is that
core_target::detach() prints "No core file now.".  That is preserved
by printing that directly from within core_file_command if not
debugging a core.

gdb/ChangeLog:
2018-05-11  Pedro Alves  <palves@redhat.com>

	* corefile.c (core_file_command): Move to corelow.c.
	* corelow.c (the_core_target): Delete.
	(core_file_command): Moved from corefile.c.  Check exec_bfd
	instead of the_core_target.  Use target_detach instead of calling
	into the_core_target directly.
	(maybe_say_no_core_file_now): New.
	(core_target::detach): Use it.
	(_initialize_corelow): Remove references to the_core_target.
	* gdbcore.h (the_core_target): Delete.