Introduce show_debug_regs

This commit adds a new global flag show_debug_regs to common-debug.h
to replace the flag debug_hw_points used by gdbserver and by the
Linux x86 and AArch64 ports, and to replace the flag maint_show_dr
used by the Linux MIPS port.

Note that some debug printing in the AArch64 port was enabled only if
debug_hw_points > 1 but no way to set debug_hw_points to values other
than 0 and 1 was provided; that code was effectively dead.  This
commit enables all debug printing if show_debug_regs is nonzero, so
the AArch64 output will be more verbose than previously.

gdb/ChangeLog:

	* common/common-debug.h (show_debug_regs): Declare.
	* common/common-debug.c (show_debug_regs): Define.
	* aarch64-linux-nat.c (debug_hw_points): Don't define.  Replace
	all uses with show_debug_regs.  Replace all uses that considered
	debug_hw_points as a multi-value integer with straight boolean
	uses.
	* x86-nat.c (debug_hw_points): Don't define.  Replace all uses
	with show_debug_regs.
	* nat/x86-dregs.c (debug_hw_points): Don't declare.  Replace
	all uses with show_debug_regs.
	* mips-linux-nat.c (maint_show_dr): Don't define.  Replace all
	uses with show_debug_regs.

gdb/gdbserver/ChangeLog:

	* server.h (debug_hw_points): Don't declare.
	* server.c (debug_hw_points): Don't define.  Replace all uses
	with show_debug_regs.
	* linux-aarch64-low.c (debug_hw_points): Don't define.  Replace
	all uses with show_debug_regs.
Introduce target/target.h

This introduces target/target.h.  This file declares some functions
that the shared code can use and that clients must implement.  It also
changes some shared code to use these functions.

gdb/ChangeLog:

	* target/target.h: New file.
	* Makefile.in (HFILES_NO_SRCDIR): Add target/target.h.
	* target.h: Include target/target.h.
	(target_read_memory, target_write_memory): Don't declare.
	* target.c (target_read_uint32): New function.
	* common/agent.c: Include target/target.h.
	[!GDBSERVER]: Don't include target.h.
	(helper_thread_id): Type changed to uint32_t.
	(agent_get_helper_thread_id): Use target_read_uint32.
	(agent_run_command): Always use target_read_memory and
	target_write_memory.
	(agent_capability): Type changed to uint32_t.
	(agent_capability_check): Use target_read_uint32.

gdb/gdbserver/ChangeLog:

	* target.h: Include target/target.h.
	* target.c (target_read_memory, target_read_uint32)
	(target_write_memory): New functions.
Introduce target_{stop,continue}_ptid

This commit introduces two new functions to stop and restart target
processes that shared code can use and that clients must implement.
It also changes some shared code to use these functions.

gdb/ChangeLog:

	* target/target.h (target_stop_ptid, target_continue_ptid):
	Declare.
	* target.c (target_stop_ptid, target_continue_ptid): New
	functions.
	* common/agent.c [!GDBSERVER]: Don't include infrun.h.
	(agent_run_command): Always use target_stop_ptid and
	target_continue_ptid.

gdb/gdbserver/ChangeLog:

	* target.c (target_stop_ptid, target_continue_ptid): New
	functions.