Implement vFile:setfs in gdbserver

Implement vFile:setfs in gdbserver

This commit implements the "vFile:setfs" packet in gdbserver.

gdb/gdbserver/ChangeLog:

	* target.h (struct target_ops) <multifs_open>: New field.
	<multifs_unlink>: Likewise.
	<multifs_readlink>: Likewise.
	* linux-low.c (nat/linux-namespaces.h): New include.
	(linux_target_ops): Initialize the_target->multifs_open,
	the_target->multifs_unlink and the_target->multifs_readlink.
	* hostio.h (hostio_handle_new_gdb_connection): New declaration.
	* hostio.c (hostio_fs_pid): New static variable.
	(hostio_handle_new_gdb_connection): New function.
	(handle_setfs): Likewise.
	(handle_open): Use the_target->multifs_open as appropriate.
	(handle_unlink): Use the_target->multifs_unlink as appropriate.
	(handle_readlink): Use the_target->multifs_readlink as
	appropriate.
	(handle_vFile): Handle vFile:setfs packets.
	* server.c (handle_query): Call hostio_handle_new_gdb_connection
	after target_handle_new_gdb_connection.