S390: Multi-inferior watchpoint support

S390: Multi-inferior watchpoint support

Support different sets of watchpoints in multiple inferiors.

gdb/ChangeLog:

	* s390-linux-nat.c (watch_areas): Remove variable.  Replace by a
	member of...
	(struct s390_debug_reg_state): ...this.  New struct.
	(struct s390_process_info): New struct.
	(s390_process_list): New variable.
	(s390_find_process_pid, s390_add_process, s390_process_info_get)
	(s390_get_debug_reg_state): New functions.
	(s390_stopped_by_watchpoint): Now access the watch_areas VEC via
	s390_get_debug_reg_state.
	(s390_prepare_to_resume): Likewise.
	(s390_insert_watchpoint): Likewise.
	(s390_remove_watchpoint): Likewise.
	(s390_forget_process, s390_linux_new_fork): New linux_nat target
	methods.
	(_initialize_s390_nat): Register them.