Add common write_memory and read_memory NetBSD routines
Instead of sharing the native-only code with all BSDs with slightly
different semantics of the kernels, share the NetBSD-only behavior beteen
the NetBSD native and gdbserver setup.
NetBSD does not differentiate the address space I and D in the
operations (contrary to OpenBSD). NetBSD handles EACCES that integrates
with NetBSD specific PaX MPROTECT error handling.
Add a verbose message in the native client that an operation could be
cancelled due to PaX MPROTECT setup.