S390: Avoid direct access to lwp_info structure

S390: Avoid direct access to lwp_info structure

When using the lwp_info structure, avoid accessing its members directly,
and use the advertised function interfaces instead.  This is according
to the instructions in linux-nat.h and prepares for making some of the
code common between gdb and gdbserver.

gdb/ChangeLog:

	* s390-linux-nat.c (s390_prepare_to_resume): Use advertised lwp
	functions instead of accessing lwp_info structure members.
	(s390_mark_per_info_changed): New function.
	(s390_new_thread): Use it.
	(s390_refresh_per_info_cb): New function.
	(s390_refresh_per_info): Remove parameter.  Refresh all lwps of
	the current process.
	(s390_insert_watchpoint): Adjust call to s390_refresh_per_info.
	(s390_remove_watchpoint): Likewise.