Disable get_ptrace_pid for NetBSD

Disable get_ptrace_pid for NetBSD

Unlike most other Operating Systems, NetBSD tracks both pid and lwp.
The process id on NetBSD is stored always in the pid field of ptid.

gdb/ChangeLog:

	* inf-ptrace.h: Disable get_ptrace_pid on NetBSD.
	* inf-ptrace.c: Likewise.
	* (gdb_ptrace): Add.
	* (inf_ptrace_target::resume): Update.
	* (inf_ptrace_target::xfer_partial): Likewise.
	* (inf_ptrace_peek_poke): Change argument `pid' to `ptid'.
	* (inf_ptrace_peek_poke): Update.