Introduce target/target.h

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.