Avoid get_ptrace_pid() usage on NetBSD in x86-bsd-nat.c

Add gdb_ptrace() that wraps the ptrace(2) API and correctly passes
the pid,lwp pair to the calls on NetBSD; and the result of
get_ptrace_pid() on other BSD Operating Systems.

gdb/ChangeLog:

	* x86-bsd-nat.c (gdb_ptrace): New.
	* (x86bsd_dr_set): Add new argument `ptid'.
	* (x86bsd_dr_get, x86bsd_dr_set, x86bsd_dr_set_control,
	x86bsd_dr_set_addr): Update.