gdbserver: turn target ops 'multifs_{open, readlink, unlink}' into methods

gdbserver: turn target ops 'multifs_{open, readlink, unlink}' into methods

gdbserver/ChangeLog:
2020-02-20  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	Turn process_stratum_target's multifs_open, multifs_readlink,
	multifs_unlink ops into methods of process_target.

	* target.h (struct process_stratum_target): Remove the target ops.
	(class process_target): Add the target ops.  Also add
	'supports_multifs'.
	* target.cc: Include "fcntl.h", "unistd.h", "sys/types.h", and
	"sys/stat.h".
	(process_target::supports_multifs): Define.
	(process_target::multifs_open): Define.
	(process_target::multifs_readlink): Define.
	(process_target::multifs_unlink): Define.

	Update the derived classes and callers below.

	* hostio.cc (handle_setfs): Update.
	(handle_open): Update.
	(handle_unlink): Update.
	(handle_readlink): Update.
	* linux-low.cc (linux_target_ops): Update.
	(linux_process_target::supports_multifs): Define.
	(linux_process_target::multifs_open): Define.
	(linux_process_target::multifs_readlink): Define.
	(linux_process_target::multifs_unlink): Define.
	* linux-low.h (class linux_process_target): Update.
	* lynx-low.cc (lynx_target_ops): Update.
	* nto-low.cc (nto_target_ops): Update.
	* win32-low.cc (win32_target_ops): Update.