pid: use PROT_READ/MAP_SHARED for readonly maps

We used to have PROT_READ | PROT_WRITE and MAP_PRIVATE for readonly
maps. However, this would reserve copy-on-write space and make the
service OOM. We could also use MAP_NORESERVE to avoid committing for
that much memory in advance, but we really don't need to have writable
space at all, so we just set the prot flags properly.

Fix T2492